Move onKeyguardGoingAway outside ActivityManager lock

The other events of WallpaperManagerInternal (onDisplayReady,
onScreenTurningOn, onScreenTurnedOn) are also called outside lock,
and only grab the WallpaperManagerService lock.

We now do the same with onKeyguardGoingAway. This does not create new
race issues: the event sent to the engine is anyway async.

This CL fixes a deadlock: see the associated bug.

Bug: 282786193
Test: treehugger
Change-Id: I8c15fec587c6124ee5aadac46b8a0e1b93507921