Add support for multiple animation tracks

The shell-transition player "Transitions" only supported
playing one animation at a time. It supported limited
support for concurrent animation via `merging`, but this
is for specific/curated situations. Since support via
merge is complicated, there weren't many implementors and
instead, by default, animations would just jump-to-end
to allow the next animation to start immediately (to
minimize perceived latency).

Unfortunately, if there are TRULY independent transitions,
this mechanism is unweildy as it'd require adding support
for any incoming transition into any active animation.

This CL moves the existing queue/merge mechanism into a
"track" and then adds support for multiple tracks to play
simultaneously. This way, for transitions which aren't
independent, the mechanism doesn't change; however, for
truly independent transitions, their corresponding
animations can also run independently.

The expectation is for WMCore to assign track ids to
transitions. Then the player (Transitions.java) can
use this information to either play them in parallel
or, in the future, do some type of merging on its own.

The default is that, all transitions with the same
track-id will play in the same track and serialize
with eachother -- but otherwise the tracks are independent.

There may, however, be some situations where a transition
might conflict with more than 1 track. In this case,
we just fall-back to a global "SYNC" and basically wait/
flush all the running animations/tracks before starting.
Supporting anything fancier is not worth the effort since
this situation isn't likely to be very common.

This SYNC is actually implemented by just generalizing the
existing SLEEP failsafe mechanic. We now just treat an
incoming SLEEP the same as SYNC.

Bug: 277838915
Bug: 264536014
Test: atest ShellTransitionTests
Test: this change, alone, should be a no-op so existing tests too.
Change-Id: I97ca21e0917be884cac105a6cb2d2c656f0e4207