Run the ondrawn Keyguard callback in the bg thread

This should give some unfold latency improvements, as the screen is on only when the pending tasks registered in ScreenOnCoordinator are completed.

If there are some long running computations in the main thread, calling onDrawn.run() will result in delays.

Flag: ACONFIG enable_background_keyguard_ondrawn_callback DISABLED
Bug: 295873557
Test: ScreenOnCoordinatorTest
Change-Id: Ie0202eb96d7b155f0f3be90dc35a3b0ae9b8e8e3