Don't update the genID for an immutable bitmap

AndroidBitmap_unlockPixels is the only way to release the ref on
pixels acquired using AndroidBitmap_lockPixels.  Apps that lock an
immutable bitmap therefore are forced to call unlock pixels. Prior
to this CL the unlock would update the GenID of the bitmap and also
print a warning that the app was modifying an immutable bitmap. After
this CL the bitmap's GenID will not be updated and no warning will be
printed.

As a result, apps that were using the NDK to allow them to mutate
immutable bitmaps will break without getting a warning message.

Test: hwui_unit_tests
Bug: 150823341
Change-Id: Ie5e5cf86a7a83d53c179c9b3cf9be1a0566cfd93