Use default FlutterActivity in non-plugin examples (#9770)

Brings hello_world and flutter_gallery Android code in line with the
corresponding iOS code.

dev/devicelab/lib/tasks/perf_tests.dart

@@ -65,7 +65,7 @@ TaskFunction createGalleryBackButtonMemoryTest() {
   return new AndroidBackButtonMemoryTest(
     '${flutterDirectory.path}/examples/flutter_gallery',
     'io.flutter.examples.gallery',
-    'io.flutter.examples.gallery.MainActivity',
+    'io.flutter.app.FlutterActivity',
   );
 }
 

examples/flutter_gallery/android/app/src/main/AndroidManifest.xml

@@ -22,7 +22,7 @@
          additional functionality it is fine to subclass or reimplement
          FlutterApplication and put your custom class here. -->
     <application android:name="io.flutter.app.FlutterApplication" android:label="Gallery" android:icon="@mipmap/ic_launcher">
-        <activity android:name=".MainActivity"
+        <activity android:name="io.flutter.app.FlutterActivity"
                   android:launchMode="singleTop"
                   android:theme="@android:style/Theme.Light.NoTitleBar"
                   android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"

examples/flutter_gallery/android/app/src/main/java/io/flutter/examples/gallery/MainActivity.java

-package io.flutter.examples.gallery;
-
-import android.os.Bundle;
-import io.flutter.app.FlutterActivity;
-import io.flutter.plugins.PluginRegistry;
-
-public class MainActivity extends FlutterActivity {
-    PluginRegistry pluginRegistry;
-
-    @Override
-    protected void onCreate(Bundle savedInstanceState) {
-        super.onCreate(savedInstanceState);
-        pluginRegistry = new PluginRegistry();
-        pluginRegistry.registerAll(this);
-    }
-}

examples/hello_world/android/app/src/main/AndroidManifest.xml

@@ -22,7 +22,7 @@
          additional functionality it is fine to subclass or reimplement
          FlutterApplication and put your custom class here. -->
     <application android:name="io.flutter.app.FlutterApplication" android:label="hello_world" android:icon="@mipmap/ic_launcher">
-        <activity android:name=".MainActivity"
+        <activity android:name="io.flutter.app.FlutterActivity"
                   android:launchMode="singleTop"
                   android:theme="@android:style/Theme.Black.NoTitleBar"
                   android:configChanges="orientation|keyboardHidden|keyboard|screenSize|locale|layoutDirection"

examples/hello_world/android/app/src/main/java/io/flutter/examples/hello_world/MainActivity.java

-package io.flutter.examples.hello_world;
-
-import android.os.Bundle;
-import io.flutter.app.FlutterActivity;
-import io.flutter.plugins.PluginRegistry;
-
-public class MainActivity extends FlutterActivity {
-    PluginRegistry pluginRegistry;
-
-    @Override
-    protected void onCreate(Bundle savedInstanceState) {
-        super.onCreate(savedInstanceState);
-        pluginRegistry = new PluginRegistry();
-        pluginRegistry.registerAll(this);
-    }
-}