Handle bubble transitions if the launch is collected into another transition

- In this particular flow, the opening of the next bubble task is
  collected into the transition for the closing of the trampoline task
  so we don't get a chance to handle the transition at request time.
  In this case, we should check during startAnimation() for the launch
  of the next bubble task, and handle the transition if possible.

Bug: 428855873
Flag: com.android.wm.shell.enable_create_any_bubble
Test: atest DefaultMixedHandlerTest

Change-Id: I880ed7110bffa67011d6966e9e0ddf7359cffcf7