Do not pause rendering when android activity loses focus (flutter/engine#4848)

* do not pause rendering when android view loses focus

engine/src/flutter/lib/ui/window.dart

@@ -47,10 +47,14 @@ enum AppLifecycleState {
   /// in the foreground inactive state. Apps transition to this state when in
   /// a phone call, responding to a TouchID request, when entering the app
   /// switcher or the control center, or when the UIViewController hosting the
-  /// Flutter app is transitioning. Apps in this state should assume that they
-  /// may be [paused] at any time.
+  /// Flutter app is transitioning.
   ///
-  /// On Android, this state is currently unused.
+  /// On Android, this corresponds to an app or the Flutter host view running
+  /// in the foreground inactive state.  Apps transition to this state when
+  /// another activity is focused, such as a split-screen app, a phone call,
+  /// a picture-in-picture app, a system dialog, or another window.
+  /// 
+  /// Apps in this state should assume that they may be [paused] at any time.
   inactive,
 
   /// The application is not currently visible to the user, not responding to

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

@@ -99,6 +99,12 @@ public class FlutterActivity extends Activity implements FlutterView.Provider, P
             super.onBackPressed();
         }
     }
+    
+    @Override
+    protected void onStop() {
+        eventDelegate.onStop();
+        super.onStop();
+    }
 
     @Override
     protected void onPause() {

engine/src/flutter/shell/platform/android/io/flutter/app/FlutterActivityDelegate.java

@@ -218,6 +218,11 @@ public final class FlutterActivityDelegate
         }
     }
 
+    @Override
+    public void onStop() {
+        flutterView.onStop();
+    }
+
     @Override
     public void onPostResume() {
         if (flutterView != null) {

engine/src/flutter/shell/platform/android/io/flutter/app/FlutterActivityEvents.java

@@ -54,6 +54,11 @@ public interface FlutterActivityEvents
      */
     void onDestroy();
 
+    /**
+     * @see android.app.Activity#onStop()
+     */
+    void onStop();
+    
     /**
      * Invoked when the activity has detected the user's press of the back key.
      *

engine/src/flutter/shell/platform/android/io/flutter/app/FlutterFragmentActivity.java

@@ -98,6 +98,13 @@ public class FlutterFragmentActivity
         }
     }
 
+    @Override
+    protected void onStop() {
+        eventDelegate.onStop();
+        super.onStop();
+    }
+
+
     @Override
     protected void onPause() {
         super.onPause();

engine/src/flutter/shell/platform/android/io/flutter/view/FlutterNativeView.java

@@ -128,7 +128,14 @@ public class FlutterNativeView implements BinaryMessenger {
 
     // Called by native to send us a platform message.
     private void handlePlatformMessage(final String channel, byte[] message, final int replyId) {
-        assertAttached();
+        // The platform may not be attached immediately in certain cases where a new bundle is run -
+        // the native view is created in a separate thread. This mostly happens when the app restarts in dev
+        // mode when switching into split-screen mode. Preventing app restarts on layout and density
+        // changes will prevent this, and afterwards this can be changed back to an assert.
+        if (!isAttached()) {
+            Log.d(TAG, "PlatformView is not attached");
+            return;
+        }
         BinaryMessageHandler handler = mMessageHandlers.get(channel);
         if (handler != null) {
             try {

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

@@ -267,7 +267,7 @@ public class FlutterView extends SurfaceView
     }
 
     public void onPause() {
-        mFlutterLifecycleChannel.send("AppLifecycleState.paused");
+        mFlutterLifecycleChannel.send("AppLifecycleState.inactive");
     }
 
     public void onPostResume() {
@@ -278,7 +278,7 @@ public class FlutterView extends SurfaceView
     }
 
     public void onStop() {
-        mFlutterLifecycleChannel.send("AppLifecycleState.suspending");
+        mFlutterLifecycleChannel.send("AppLifecycleState.paused");
     }
 
     public void onMemoryPressure() {