Scaffolding for Shell Transitions

Initial rough implementation of Shell Transitions.

Shell Transitions can be enabled by setting ENABLE_SHELL_TRANSITIONS
via `adb shell setprop persist.debug.shell_transit 1`

The main structure behind this is:

- TransitionController in Core responsible for handling the
collection of transition participants and communicating with
the TransitionPlayer in Shell.

- Transitions in Shell responsible for starting transitions
and playing associated animations.

This initial version only supports task open/show/close and does
a simple fade. The flow is:
1. activityStarter calls to Transitions.requestStartTransition
2. Transitions will prepare to start and then call
   WindowOrganizerController.startTransition().
2. Various operations in Core call TransitionController.collect()
   to track participants. This collect() acts as a replacement for
   adding to mOpening/ClosingApps.
3. The legacy executeAppTransition() is wired into
   TransitionController.setReady() for now. This signal
   is used to ready a BLASTSyncEngine.
4. When all participants are done drawing, TransitionController
   will commit any showing visibility changes and then will construct
   a TransitionInfo object summarizing all the changes needed for
   animation. This gets sent to Transitions.onTransitionReady()
5. Transitions then plays the transition animation and on
   completion will call WindowOrganizerController.finishTransition()
   which will tell TransitionController to do any finishing
   work (like commiting deferred hide changes).

Bug: 161980187
Test: enable the flag and try opening/closing tasks. Also added
      TransitionControllerTests
Change-Id: I80c3a241fc12004a894c26523a1e5828e0c533ff

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