Move LowLightMonitor operations to background thread.

LowLightMonitor contains a number of callbacks that are triggered by
events such as screen turning on and off (triggered during unfolding as
well). This changelist moves these operations to a background thread to
not block the main thread.

Test: atest LowLightMonitorTest
Flag: EXEMPT bugfix
Fixes: 394520234
Change-Id: I435c21e2c1e0827c8de24cd0737cfa55c1fab403