Allow FirstFrameListeners to remove themselves from the FlutterView's…

… list (flutter#4871)

Fixes flutter/flutter#15884

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

@@ -767,7 +767,9 @@ public void updateSemantics(ByteBuffer buffer, String[] strings) {
 
     // Called by native to notify first Flutter frame rendered.
     public void onFirstFrame() {
-        for (FirstFrameListener listener : mFirstFrameListeners) {
+        // Allow listeners to remove themselves when they are called.
+        List<FirstFrameListener> listeners = new ArrayList<>(mFirstFrameListeners);
+        for (FirstFrameListener listener : listeners) {
             listener.onFirstFrame();
         }
     }