Add onStart hook to FlutterFragmentActivity (flutter/engine#6719)

Seems like this file was missed when onStart hooks were added recently in this pull request: https://github.com/flutter/engine/pull/5177

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

@@ -101,6 +101,12 @@ public class FlutterFragmentActivity
             super.onBackPressed();
         }
     }
+        
+    @Override
+    protected void onStart() {
+        super.onStart();
+        eventDelegate.onStart();
+    }
 
     @Override
     protected void onStop() {