Bump APK version in prep for release

@abarth

packages/flutter/example/demo_launcher/apk/AndroidManifest.xml

@@ -3,7 +3,7 @@
      Use of this source code is governed by a BSD-style license that can be
      found in the LICENSE file.
  -->
-<manifest xmlns:android="http://schemas.android.com/apk/res/android" package="org.domokit.sky.demo" android:versionCode="20" android:versionName="0.0.20">
+<manifest xmlns:android="http://schemas.android.com/apk/res/android" package="org.domokit.sky.demo" android:versionCode="21" android:versionName="0.0.21">
 
     <uses-sdk android:minSdkVersion="14" android:targetSdkVersion="21" />
     <uses-permission android:name="android.permission.INTERNET" />

packages/flutter/example/demo_launcher/apk/release_notes/0.0.21.txt

+Huge speed improvements in Asteroids.
+Fixed startup crash on many devices.
+Lots of various other widget fixes.