[Android] update tutorial and provide a SDK setup tutorial

examples/demo-apps/android/LlamaDemo/README.md

@@ -3,12 +3,18 @@
 This app demonstrates the use of the LLaMA chat app demonstrating local inference use case with ExecuTorch.
 
 ## Prerequisites
-* Refer to [Setting up ExecuTorch](https://pytorch.org/executorch/stable/getting-started-setup) to set up the repo and dev environment.
-* Download and install [Android Studio and SDK](https://developer.android.com/studio).
+* Set up your ExecuTorch repo and environment if you haven’t done so by following the [Setting up ExecuTorch](https://pytorch.org/executorch/stable/getting-started-setup) to set up the repo and dev environment.
+* Install [Java 17 JDK](https://www.oracle.com/java/technologies/javase/jdk17-archive-downloads.html).
+* Install the [Android SDK API Level 34](https://developer.android.com/about/versions/14/setup-sdk) and
+  [Android NDK 25.0.8775105](https://developer.android.com/studio/projects/install-ndk).
+ * If you have Android Studio set up, you can install them with
+   * Android Studio Settings -> Language & Frameworks -> Android SDK -> SDK Platforms -> Check the row with API Level 34.
+   * Android Studio Settings -> Language & Frameworks -> Android SDK -> SDK Tools -> Check NDK (Side by side) row.
+ * Alternatively, you can follow [this guide](https://github.com/pytorch/executorch/blob/856e085b9344c8b0bf220a97976140a5b76356aa/examples/demo-apps/android/LlamaDemo/SDK.md) to set up Java/SDK/NDK with CLI.
 * Supported Host OS: CentOS, macOS Sonoma on Apple Silicon.
 
 ```{note}
-This demo app and tutorial has only been validated with arm64-v8a [ABI](https://developer.android.com/ndk/guides/abis), with NDK 25.
+This demo app and tutorial has only been validated with arm64-v8a [ABI](https://developer.android.com/ndk/guides/abis), with NDK 25.0.8775105.
 ```
 
 ## Getting models
@@ -28,6 +34,11 @@ The demo app searches in `/data/local/tmp/llama` for .pte and .bin files as LLAM
 ## Build JNI library
 1. Open a terminal window and navigate to the root directory of the `executorch`.
 2. Set the following environment variables:
+```{note}
+<path_to_android_ndk> is the root for the NDK, which is usually under
+~/Library/Android/sdk/ndk/XX.Y.ZZZZZ for macOS, and contains NOTICE and README.md.
+We use <path_to_android_ndk>/build/cmake/android.toolchain.cmake for CMake to cross-compile.
+```
 ```bash
 export ANDROID_NDK=<path_to_android_ndk>
 export ANDROID_ABI=arm64-v8a
@@ -40,7 +51,7 @@ popd
 ```
 This is running the shell script [setup.sh](./setup.sh) which configures the required core ExecuTorch, LLAMA2, and Android libraries, builds them, and copy to jniLibs.
 
-## Build Java app
+## Build APK
 ### Alternative 1: Android Studio (Recommended)
 1. Open Android Studio and select "Open an existing Android Studio project" to open examples/demo-apps/android/LlamaDemo.
 2. Run the app (^R). This builds and launches the app on the phone.

examples/demo-apps/android/LlamaDemo/SDK-quick-setup-guide.md

@@ -0,0 +1,94 @@
+# Guide to set up Java/SDK/NDK for Android
+
+Follow this doc if you haven't set up Java/SDK/NDK for Android development
+already.
+This doc provides a CLI tutorial to set them up. Otherwise, you can do the same
+thing with Android Studio GUI.
+
+## Set up Java 17
+1. Download the archive from Oracle website.
+Make sure you have read and agree with the terms and conditions from the website before downloading.
+```bash
+export DEV_HOME=<path-to-dev>
+cd $DEV_HOME
+```
+Linux:
+```bash
+curl https://download.oracle.com/java/17/archive/jdk-17.0.10_linux-x64_bin.tar.gz -o jdk-17.0.10.tar.gz
+```
+macOS:
+```bash
+curl https://download.oracle.com/java/17/archive/jdk-17.0.10_macos-aarch64_bin.tar.gz -o jdk-17.0.10.tar.gz
+```
+2. Unzip the archive. The directory named `jdk-17.0.10` is the Java root directory.
+```bash
+tar xf jdk-17.0.10.tar.gz
+```
+3. Set `JAVA_HOME` and update `PATH`.
+
+Linux:
+```bash
+export JAVA_HOME="$DEV_HOME"/jdk-17.0.10
+export PATH="$JAVA_HOME/bin:$PATH"
+```
+macOS:
+```bash
+export JAVA_HOME="$DEV_HOME"/jdk-17.0.10.jdk/Contents/Home
+export PATH="$JAVA_HOME/bin:$PATH"
+```
+
+Note: Oracle has tutorials for installing Java on
+[Linux](https://docs.oracle.com/en/java/javase/17/install/installation-jdk-linux-platforms.html#GUID-4A6BD592-1840-4BB4-A758-4CD49E9EE88B)
+and [macOS](https://docs.oracle.com/en/java/javase/17/install/installation-jdk-macos.html#GUID-E8A251B6-D9A9-4276-ABC8-CC0DAD62EA33).
+Some Linux distributions has JDK package in package manager. For example, Debian users can install
+openjdk-17-jdk package.
+
+## Set up Android SDK/NDK
+Android has a command line tool [sdkmanager](https://developer.android.com/tools/sdkmanager) which
+helps users managing SDK and other tools related to Android development.
+
+1. Go to https://developer.android.com/studio and download the archive from "Command line tools
+only" section. Make sure you have read and agree with the terms and conditions from the website.
+
+Linux:
+```bash
+curl https://dl.google.com/android/repository/commandlinetools-linux-11076708_latest.zip -o commandlinetools.zip
+```
+macOS:
+```bash
+curl https://dl.google.com/android/repository/commandlinetools-mac-11076708_latest.zip -o commandlinetools.zip
+```
+2. Unzip.
+```bash
+unzip commandlinetools.zip
+```
+3. Specify a root for Android SDK. For example, we can put it under `$DEV_HOME/sdk`.
+
+```
+mkdir -p $DEV_HOME/sdk
+export ANDROID_HOME="$(realpath $DEV_HOME/sdk)"
+# Install SDK 34
+./cmdline-tools/bin/sdkmanager --sdk_root="${ANDROID_HOME}" --install "platforms;android-34"
+# Install NDK
+./cmdline-tools/bin/sdkmanager --sdk_root="${ANDROID_HOME}" --install "ndk;25.0.8775105"
+# The NDK root is then under `ndk/<version>`.
+export ANDROID_NDK="$ANDROID_HOME/ndk/25.0.8775105"
+```
+
+### (Optional) Android Studio Setup
+If you want to use Android Studio and never set up Java/SDK/NDK before, or if
+you use the newly installed ones, follow these steps to set Android Studio to use
+them.
+
+Copy these output paths to be used by Android Studio
+```bash
+echo $ANDROID_HOME
+echo $ANDROID_NDK
+echo $JAVA_HOME
+```
+
+Open a project in Android Studio. In Project Structure (File -> Project
+Structure, or `⌘;`) -> SDK Location,
+* Set Android SDK Location to the path of $ANDROID_HOME
+* Set Android NDK Location to the path of $ANDROID_NDK
+* Set JDK location (Click Gradle Settings link) -> Gradle JDK -> Add JDK... to the path of $JAVA_HOME