Retain the gainmap during Bitmap.asShared()

This was getting lost in some IPCs because the gainmap of an Ashmem
Bitmap was passed through as a local native pointer instead of also
being an Ashmem object.

Test: GainmapTests
Test: GainmapTest
Test: BitmapTest
Bug: 390675155
Flag: EXEMPT bugfix
Change-Id: I1940c5eb58d1cc5a83836f77e2618bd86ca35741