Use activities' own animations, unless opaque activity on top

If there is no opaque activity A on top of activity B, the animation
options are derived from B.
If there is an opaque A over B, the animation options for B are the same
as A.

Fix: 327332488
Test: TransitionTests
Flag: com.android.window.flags.custom_animations_behind_translucent
Change-Id: I82d4d361ea7219e39c1e3db6abe6d14374cb079e