Use transparent instead of null background during Activity Transitions

Bug 32952142

When doing activity transitions, a window is temporarily translucent
during the transition. When a view hierarchy completely covers the
window, the background is never seen and applications can use a
null background to avoid overdraw. However, during the activity
transition, the underlying activity is seen during the transition.
With a null background, the buffer isn't properly cleared and
the uncleared buffer will show strange effects.

This CL forces a transparent background temporarily during the
activity transition to avoid this problem.

Test: manual testing using test application

Change-Id: I63f24dba3c2f810944bcbf07faf309f9f1c5889a