Post the onTransitionAnimationEnd() work to the main thread.

This was already submitted but reverted due to the introduction of a
flicker. For now we're only re-enabling it for return animations, which
don't display the flicker.

Bug: 330672236
Flag: EXEMPT returns are flagged at usage site
Test: atest TransitionAnimatorTest
Change-Id: I35830caefde3d5371bee7b8b020c7b4bd07076a7