Minor cleanup for ActionBar

- make WPActionBarActivity an abstract class, as well as the
  onBlogChanged method.  Update subclasses accordingly.
- fix ActionBar code in ViewWebStats that was causing the app to crash
  on older versions of Android

Fixes #412

git-svn-id: https://android.svn.wordpress.org/trunk@890 a6565c4b-a94d-4d28-bb26-aa9e5ab542c0