Default RotateDrawable's pivot set to (50%, 50%)

In order to define the pivot in an XML-instanciated RotateDrawable, android:pivotX
and android:pivotY had to be set in your XML. Forgetting to set those attributes
ended up in a NullPointerException (tv = null) that were caught by the Resources.getDrawable()
method (caught as an Exception). As a result a not-very-accurate message was logged:
"Resource not found ...". Defining a default pivot value seems like a great fix.
Some other fixes would be to modify the documentation or notify the user with a better
explanation than "Resource not found ...".