Change version to 0.21

Change-Id: I8e3792ba43293313baab0bb0d6bebba8c65df4b9

java/AndroidManifest.xml

 <manifest xmlns:android="http://schemas.android.com/apk/res/android"
         package="com.google.android.inputmethod.latin"
-        android:versionCode="20"
-        android:versionName="0.20">
+        android:versionCode="21"
+        android:versionName="0.21">
 
     <uses-sdk android:minSdkVersion="8"></uses-sdk>