Introduce new API overrideActivityTransition(1/N)

...which should be able to replace overridePendingTransition.

This provide an API set to customize activity animation, and it can be
called at any time while the activity still alive.

1. If an Activity has calls overrideActivityTransition, and it also set
another activity animation in Window#windowAnimations, system will
choose the animation which set from overrideActivityTransition.

2. The animation set from overridePendingTransition still have highest
priority when system is looking for next transition animation, but by
calling the new API overrideActivityTransition before activity lifecycle
change, system will support to play the predict-back animation if the
app has opt-in enableOnBackInvokedCallback.

Bug: 259427810
Test: atest ActivityTransitionTests
Change-Id: I33be7f7b0ee1122fd29f9f3bd6f023bb00b00ad8