Optimize activity snapshot for shell transition.

- Ignore activity snapshot for legacy transition.
  Analysis activity snapshot operations after transition finish, so
  there won't block onSurfacePlacement. Also it would be much faster by
  checking transition targets vs checking everything from participants.
- Only capture activity snapshot when transaction ready.

Bug: 259497289
Test: atest ActivitySnapshotControllerTests TransitionTests
Change-Id: I0d2ed127508abd1f85f2e8533c5c3cd6fc353a6f