Add MODIFY_AUDIO_SETTINGS_PRIVILEGED for changing assistant volume

Since the new assistant conversation mode is protected by
MODIFY_AUDIO_SETTINGS_PRIVILEGED it makes sense to allow volume changes
for STREAM_ASSISTANT with the same permission

Test: trivial
Bug: 416329698
Flag: EXEMPT bugfix
Change-Id: Id7cf33c35eb017fdd441511a89ef1c60e93262c0