audio policy: re enable setForceUse for system apps.

Some system apps (not having system AIDs) still need this
hidden API due to lack of better option for media use case routing.

Add protection by privileged permission MODIFY_AUDIO_ROUTING instead.

Bug: 79475856
Test: manual regression for music playback over speaker and bluetooth
Change-Id: Icaa585a9fa4c043c291f24c9bab4a93f02406955