Fix a deadlock due to AMS calling into WMS with its lock held.

The full deadlock cycle involved ActivityManagerService,
WindowManagerService, and DevicePolicyManagerService.

Fixed by posting the work of enabling the screen to the handler
instead of doing it immediately while holding the activity manager
lock.

Bug: 16578860
Change-Id: I10514cd2df993c7193747ae823e1990a2c34e196