Make WC#applyAnimation can apply on Task / TaskStack layer (1/N)

- Use WC#getDisplayContent instead WC#mDisplayContent in
WC#okToAnimation and WC#okToAnimate.

- Fix some places that calls isAnimating() needs to add PARENTS flag.

- Add unit tests to verify Task / Stack can applyAnimation with expected
  animation state.

- In Task#createRemoteAnimationTarget(), use WC#getTopMostActivity to
  get top most activity rather than using getTopVisibleActivity, to
  prevent closing activity may not get corresponding activity instance
  case.

- Fix WC#isWaitingForTransitionStart and WC#isAppTransition won't get
  correct result when animating app transition on Task / TaskStack container.

- To ensure onAppTransitionFinished will be called when enabling
  hierarchical animation, needs to let activity.onAnimationFinished()
  also be called.

- Implement Task#getAnimationBounds to get stack's bounds when
appStackClipMode is STACK_CLIP_BEFORE_ANIM.

Bug: 142617871
Bug: 131661052
Test: atest WindowContainerTests

Change-Id: Id97087819cab54ba35fb4b9ae4b5b95a572ad377