Refine check for transient transition

- An activity launch can happen mid-transient transition which can
  involve the transiently-hiding tasks (ie. quickswitch between
  split pairs will replace the previously paired tasks) in a new
  transition.  Since the intermediate transition itself is not
  transiently launching anything, it will still attempt to snapshot
  those hiding tasks, but if they are already a part of the
  transient transition, then their snapshots may be controled by the
  transition player (ie. Launcher), and we should not attempt to
  snapshot the tasks in a state which may have already changed.

  ie.
    Pair of task A | B in split (30/70 split ratio)
    Start transient transition T1 with A, B to back, and Home to front
    Start new split pair C | D in split (50/50 split ratio) replacing A | B
        - This resizes the split roots as a part of creating
          the new split, resulting in snapshots of A | B in
  	  50/50 instead of the original 30/70 size

    As long as Launcher requests a snapshot of A | B before finishing
    the recents transition, we will skip taking a snapshot on ready
    and also at the end of the transition (where it checks the last
    snapshot timestamp relative to transition start)

Bug: 425846640
Flag: EXEMPT bugfix
Test: atest WmTests:TransitionTests

Change-Id: Ibf5557ccfa391e60aed746989563ef93910b9e52