Fix Transition test failure.

Bug: 67049319

TransitionUtils was returning null when the View wasn't attached,
but Visibility transitions can do that intentionally. This CL
temporarily adds detached views to the view hierarchy as part of
an overlay while creating the hardware bitmap representation.

Test: ran transition CTS tests
Change-Id: Ie335619953653dce0224514f0d5c9c8eb00ee1a9