Refactor transition player to fix merge ordering

The existing code was starting to get non-understandable and had
several bugs around ordering of transitions which were merged or
aborted.

This CL separates the singular "active transitions" list into
3 queues. When the player becomes aware of a transition, it
places it into the pending queue. Once Core reports taht it is
ready, it is moved to the ready queue. If there are no active
animations (active list is empty), it will move the first item
from the ready queue into the active list and start playing it.
If there is an ongoing animation, it will attempt to merge the
first item in the ready queue -- if it succeeds, it will be
moved into the playing transition's list of "merged" transitions.
Finished transitions are removed from the active list.

Whenever a significant change happens (something finishes, merges,
becomes ready), it will re-check the ready-queue. This continues
until there is nothing left.

Keeping separate lists should make it clearer what state each
transition is in and reduces a lot of confusing logic.

Additionally, this is a better base for eventually having parallel
animaations.

Bug: 270986780
Test: ShellTransitionTests (existing and new testInterleavedMerging)
Change-Id: I84ac88416d9d152dd51a864b2a3bac61eded0b3f