Ensure move-to-back has its own transition.

In general, we need to move to a model where each transition is
a "fixed" transaction (rather than absorbing everything within
a specific time-window).

Bug: 232020248
Test: PinnedStackTests#testMovePipToBack*
Change-Id: Icf5d36e507bc70736e7760c50d5de297bfe4d8f4