AudioAttributes: handle unknown usage like media

In the native audio policy management, the default stream type
  for volume is STREAM_MUSIC, i.e. any player by default is
  considered a "media" player. Do the same for audio attributes
  with regards to suppressible behaviors.

Bug: 74052135
Test: ZenModeHelperTest + see bug
Change-Id: I8f65044807fd3e745f21e031a0aa01a5e6f74cd2