Eliminate mHasAlpha

 Bug: 13580771

 The cached value mHasAlpha on Bitmap could get out of sync with the
 real value stored in the SkBitmap. Fix this by just eliminating the
 cached value. This adds an extra JNI call in copy(), createBitmap(),
 and setPremultiplied(), none of which are going to be significantly
 impacted by the extra JNI call.

Change-Id: I4fce069a77d9b3bbb47dde04275ff8510ce7e4f0