audio policy: new getOutputForAttr() prototype.

Update getOutputForAttr() prototype and group all logic
dealing with audio attributes to stream type conversion in
audio policy manager.

getOutputForAttr():
- specifies the audio session (for future use)
- returns a status code
- receives either stream type (for legacy) or audio attributes
- returns an updated streamtype

Remove logic dealing with legacy stream types to attributes conversion
from AudioTrack.

Use correct type for audio sessions in other APIs (startOutput() ...).

releaseOutput() specifies the audio session (for future use).

Bug: 18067208.

Change-Id: I1bfbe9626c04c7955d77f8a70aecfad2cb204817