Suppress warning for the new Activity library. (flutter/engine#49046)

Context: cl/582573869

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

@@ -151,6 +151,7 @@ public class FlutterFragmentActivity extends FragmentActivity
   }
 
   @Override
+  @SuppressWarnings("MissingSuperCall")
   public void onUserLeaveHint() {
     eventDelegate.onUserLeaveHint();
   }

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

@@ -636,6 +636,7 @@ public class FlutterFragmentActivity extends FragmentActivity
   }
 
   @Override
+  @SuppressWarnings("MissingSuperCall")
   public void onUserLeaveHint() {
     flutterFragment.onUserLeaveHint();
   }