Make BitmapDrawable#setBitmap public

Bug: 154628299
Test: I20d4f94a145474394bc660368d9ddd1897001f4a

This allows displaying a new Bitmap without paying the cost of creation
and GC of the BitmapDrawable.

Non-SDK-API usage is low, but this is useful. Previously it was @hide
and used by the system. Once BitmapDrawable is in a mainline module,
the system can no longer access @hide methods.

Change-Id: I6a24b3c5870b0ee7d171aab86257896261b4b8c7