Add ability to get download id for file progress tracking.

Author: annzimmer

firebase-ml-modeldownloader/firebase-ml-modeldownloader.gradle

@@ -80,7 +80,7 @@ dependencies {
 
     testImplementation 'androidx.test:core:1.3.0'
     testImplementation 'com.github.tomakehurst:wiremock-standalone:2.26.3'
-    testImplementation "com.google.truth:truth:1.0.1"
+    testImplementation "com.google.truth:truth:$googleTruthVersion"
     testImplementation "org.robolectric:robolectric:$robolectricVersion"
     testImplementation 'com.google.truth.extensions:truth-proto-extension:1.0'
     testImplementation 'com.google.protobuf:protobuf-java-util:3.11.0'

firebase-ml-modeldownloader/src/main/java/com/google/firebase/ml/modeldownloader/internal/SharedPreferencesUtil.java

@@ -274,6 +274,12 @@ public synchronized void setCustomModelStatsCollectionEnabled(boolean enable) {
         .apply();
   }
 
+  /**
+   * Gets the start time (in ms) of the model download attempt.
+   *
+   * @param customModel model
+   * @return time in ms
+   */
   public synchronized long getModelDownloadBeginTimeMs(@NonNull CustomModel customModel) {
     return getSharedPreferences()
         .getLong(