Execute low light conditions on background thread.

This changelist ensures that low light logic occurs within the
background scope so no blocking calls are made on the main thread.
The only exception is DeviceInactiveCondition, which requires
keyguard update callbacks to be registered on the main thread.

Fixes: 394520234
Test: atest DirectBootConditionTest
Test: atest DeviceInactiveConditionTest
Test: atest ForceLowLightConditionTest
Test: atest LowLightConditionTest
Test: atest ScreenSaverEnabledConditionTest
Flag: EXEMPT bugfix
Change-Id: Iab9dd2ae99ef698ee185b148634807f2503b0143