Removed the @hide annotation on various StateListDrawable APIs to better

match the API surface exposed in ColorStateList.

Removed the @hide annotation for the following StateListDrawable APIs:
--getStateCount
--getStateSet
--getStateDrawable
--getStateDrawableIndex

Fixes: 30325322
Test: Re-ran StateListDrawable CTS tests and added tests to verify each
of the newly exposed APIs

Change-Id: If93a017b6cf690fc38381a234bf3bd19d0bfc675