Update PowerManager to allow dreaming when postured

This introduces a new user-configurable triggering mechanism for
dreaming - posturing. Posturing is a physical property of the device
itself - defined as the device being stationary +
upright. When this occurs, we allow dreaming if the user has configured
it.

Bug: 383208131
Test: atest PowerManagerServiceTest
Flag: android.service.dreams.allow_dream_when_postured
Change-Id: Ie372b4ad06d7567c55c62f31dcedc6bbb076bff6