Fix PiP transition ordering

First, this cancels the existing pip animation if it is
told to exit. Without this, an exit animation that
starts before an enter animation finishes will have its
state clobbered by the exit's finishTransition call.

Next, make pip-enter transitions queue-up so that they
don't interleave before other queued transitions (very
common during CTS).

Bug: 231150615
Test: atest PinnedStackTests
Change-Id: I7f162c0b1f24ab845770b805c8fae2252814945e