Add posturing dream trigger to Settings UI

Adds ability for users to select posturing as the trigger for when
dreams will be shown.

Bug: 383208131
Test: locally - mp droid and verified posturing option is displayed in
Settings
Flag: android.service.dreams.allow_dream_when_postured

Change-Id: Iae69b9e488181b1e2af7474e3e9879a92b51274c