Updated DrawableWrapper implementation to support the hidden setXfermode
method to delete the corresponding Xfermode to the wrapped drawable

Added coretest to verify the hidden API properly propagates the Xfermode
to the wrapped drawable

Change-Id: I1f14105b7019b57bfcdd7182bb50ea2465c91ffe
Fixes: 30840201
Test: Ran atest on coretests
2 files changed