Do not start pending animation if another transition is collecting.

If another transition is collecting, something else is happening and
the BackTransitionHandler cannot handle the transition, so cancel the
predictive back animation to allow the collecting transition to proceed.

Flag: EXEMPT bugfix
Bug: 376582378
Test: presubmit
Change-Id: Ie8a3bb25003d04a1758431707b291552c33925a4