Find a background to animate when launching a dialog or activity

This CL makes the launch animations a bit more robust by trying to find
a background in the View that is expanded into the activity/dialog
instead of just taking the View background (that can sometimes be null).

See b/222668708#comment1 and #comment2 for before/after videos.

Bug: 222668708
Test: Manual
Change-Id: I590b6d77a7c9b29a73f37e1f8072643cc8c0bcfa