Send screen events to both engines

Changes are only under our flag. If there are two active engines,
notify both for the following events:
- setInAmbientMode (only sent if ImageWallpaper or supportsAmbientMode
  is true)
- notifyWakingUp
- notifyGoingToSleep
- notifyScreenTurnedOn
- notifyScreenTurningOn

Bug: 253507223
Test: Manual (log the events with two different live wallpapers)
Change-Id: I1ebbafaca39ce875d2b8e31da986d26f55e45de4