audio policy: add forced uses for system

The volume and routing policy of AUDIO_STREAM_ENFORCED_AUDIBLE is
now controlled by AudioService by means of new forced use type
AUDIO_POLICY_FORCE_FOR_SYSTEM with config
AUDIO_POLICY_FORCE_SYSTEM_ENFORCED.

Bug 7032634.

Change-Id: Ib1834ec507f3aa726895b8fd3e8a91cc9cc601a7