Low light display behavior settings.

This changelist introduces two new settings to control low light
display behavior. An integer setting stores the user preference
of one of the following options:
- nothing (default unset value)
- no dreaming
- turn display off
- low light clock dream

Additionally, a boolean stores whether the setting is enabled.

Test: EXEMPT setting value introduction
Bug: 408229468
Flag: android.os.low_light_dream_behavior
Change-Id: I4a86b9fcfedfef3ee0621e01ea365cb36bd10e15