Fix some Javadoc warnings (flutter#3391)

shell/platform/android/io/flutter/app/FlutterActivity.java

@@ -146,6 +146,7 @@ public boolean loadIntent(Intent intent) {
     /**
      * Returns the Flutter view used by this activity, may be null before
      * onCreate was called.
+     * @return The FlutterView.
      */
     public FlutterView getFlutterView() {
       return flutterView;

shell/platform/android/io/flutter/view/FlutterMain.java

@@ -121,6 +121,7 @@ public String getLogTag() {
 
         /**
          * Set the tag associated with Flutter app log messages.
+         * @param tag Log tag.
          */
         public void setLogTag(String tag) {
             logTag = tag;
@@ -129,13 +130,16 @@ public void setLogTag(String tag) {
 
     /**
      * Starts initialization of the native system.
+     * @param applicationContext The Android application context.
      */
     public static void startInitialization(Context applicationContext) {
         startInitialization(applicationContext, new Settings());
     }
 
     /**
      * Starts initialization of the native system.
+     * @param applicationContext The Android application context.
+     * @param settings Configuration settings.
      */
     public static void startInitialization(Context applicationContext, Settings settings) {
         sSettings = settings;
@@ -158,6 +162,8 @@ public static void startInitialization(Context applicationContext, Settings sett
 
     /**
      * Blocks until initialization of the native system has completed.
+     * @param applicationContext The Android application context.
+     * @param args Flags sent to the Flutter runtime.
      */
     public static void ensureInitializationComplete(Context applicationContext, String[] args) {
         if (sInitialized) {

shell/platform/android/io/flutter/view/FlutterView.java

@@ -519,7 +519,10 @@ public void run() {
         }
     }
 
-    /** Return the most recent frame as a bitmap. */
+    /**
+     * Return the most recent frame as a bitmap.
+     * @return A bitmap.
+     */
     public Bitmap getBitmap() {
         return nativeGetBitmap(mNativePlatformView);
     }
@@ -741,6 +744,9 @@ private boolean handleAccessibilityHoverEvent(MotionEvent event) {
      * Send a message to the Flutter application. The Flutter application can
      * register a platform message handler that will receive these messages with
      * the PlatformMessages object.
+     * @param channel Name of the channel that will receive this message.
+     * @param message Message payload.
+     * @param callback Callback that receives a reply from the application.
      */
     public void sendPlatformMessage(String channel, String message, MessageReplyCallback callback) {
         int responseId = 0;
@@ -754,6 +760,9 @@ public void sendPlatformMessage(String channel, String message, MessageReplyCall
     /**
      * Send a message to the Flutter application.  The Flutter Dart code can register a
      * host message handler that will receive these messages.
+     * @param channel Name of the channel that will receive this message.
+     * @param message Message payload.
+     * @param callback Callback that receives a reply from the application.
      */
     public void sendToFlutter(String channel, String message, MessageReplyCallback callback) {
         sendPlatformMessage(channel, message, callback);
@@ -771,6 +780,8 @@ public interface MessageReplyCallback {
     /**
      * Register a callback to be invoked when the Flutter application sends a message
      * to its host.
+     * @param channel Name of the channel used by the application.
+     * @param listener Called when messages arrive.
      */
     public void addOnMessageListener(String channel, OnMessageListener listener) {
         mOnMessageListeners.put(channel, listener);
@@ -779,6 +790,8 @@ public void addOnMessageListener(String channel, OnMessageListener listener) {
     /**
      * Register a callback to be invoked when the Flutter application sends a message
      * to its host.  The reply to the message can be provided asynchronously.
+     * @param channel Name of the channel used by the application.
+     * @param listener Called when messages arrive.
      */
     public void addOnMessageListenerAsync(String channel, OnMessageListenerAsync listener) {
         mAsyncOnMessageListeners.put(channel, listener);
@@ -787,6 +800,8 @@ public void addOnMessageListenerAsync(String channel, OnMessageListenerAsync lis
     public interface OnMessageListener {
         /**
          * Called when a message is received from the Flutter app.
+         * @param view The Flutter view hosting the app.
+         * @param message Message payload.
          * @return the reply to the message (can be null)
          */
         String onMessage(FlutterView view, String message);
@@ -795,6 +810,8 @@ public interface OnMessageListener {
     public interface OnMessageListenerAsync {
         /**
          * Called when a message is received from the Flutter app.
+         * @param view The Flutter view hosting the app.
+         * @param message Message payload.
          * @param response Used to send a reply back to the app.
          */
         void onMessage(FlutterView view, String message, MessageResponse response);