Make FLAG_KEEP_SCREEN_ON use per-display wakelocks

This moves from having a single global wakelock held by
WindowManagerService whenever any window on any display has
FLAG_KEEP_SCREEN_ON set, to having one per DisplayContent that is only
held when a window on that Display has the flag set.

Bug: 185157739
Test: atest WmTests
Change-Id: I82dcb693ad2bea0a243be4299dae836a1f53930c