Debounce dreaming signal in LowLightBehaviorCoreStartable.

This changelist ensures that intermediate dream states do not affect
the low light monitoring state. This protects against temporary, short
interruptions in the dream state from events such as switching between
dreams.

Fixes: 440661063
Test: atest LowLightBehaviorCoreStartableTest#testDreamDebounce
Flag: EXEMPT bugfix
Change-Id: I11d139ab11972b05a87e2e8f4507b54ab62193a1