Add posturing noop implementation

The posturing signal is device-specific, so we add a reference noop
implementation which can be overridden on a per-device basis.

Bug: 383208131
Flag: android.service.dreams.allow_dream_when_postured
Test: atest PosturingInteractorTest
Change-Id: I18b22814d75f23a732061fa591f566f61e5a62b8