Replace onTaskMovedToFront with a transition observer in bubbles

The TaskStackListenerCallback is not synchronized with shell transit
which results in this bug. Instead of using it, use a Transition
Observer to see when tasks are opening / moving to front.

Test: atest BubblesTransitionObserverTest BubblesTest
Test: - have more than 1 bubble and have a telegram bubble
      - hit the 'x' in telegram UI to finish the activity
      => observe that the next bubble is selected
Bug: 296821517
Change-Id: I22afa4fb720b80e4ed9d7e6775bfe6c4a3f2e91c