Add SystemApi for querying audio routing

  Add a new @SystemApi for querying audio playback routing for
a given AudioAttributes.
  Note that unlike the (@UnsupportedUsage) getDevicesForStream API,
this method takes into account any current dynamic policy (unless
it is based on uid).

Bug: 144440677
Test: atest AudioServiceHostTest
Change-Id: I0431e792a209865ffa771a521c2f0792c07ce5d4