[libc++][Android] Enable Android testing in BuildKite CI

Enable testing for two NDK configurations:
 - android-ndk-21-def-x86
 - android-ndk-33-goog-x86_64

Fixes: #69270

Author: rprichard

libcxx/utils/ci/buildkite-pipeline.yml

@@ -1116,3 +1116,33 @@ steps:
           - exit_status: -1  # Agent was lost
             limit: 2
       timeout_in_minutes: 120
+
+  - group: ":android: Android"
+    steps:
+    - label: "Android 5.0, x86 NDK"
+      command: "libcxx/utils/ci/run-buildbot android-ndk-21-def-x86"
+      artifact_paths:
+        - "**/test-results.xml"
+        - "**/*.abilist"
+      agents:
+        queue: "libcxx-builders"
+        os: "android"
+      retry:
+        automatic:
+          - exit_status: -1  # Agent was lost
+            limit: 2
+      timeout_in_minutes: 120
+
+    - label: "Android 13, x86_64 NDK"
+      command: "libcxx/utils/ci/run-buildbot android-ndk-33-goog-x86_64"
+      artifact_paths:
+        - "**/test-results.xml"
+        - "**/*.abilist"
+      agents:
+        queue: "libcxx-builders"
+        os: "android"
+      retry:
+        automatic:
+          - exit_status: -1  # Agent was lost
+            limit: 2
+      timeout_in_minutes: 120