audio policy: add deep audio buffering flag.

Add a value in audio policy flags to indicate an output
stream providing deep audio PCM buffers. Such an output
helps reducing the power consumption at the expense of audio latency.

Stagefright will indicate a preference for such an output when playing music
only (no video) and the audio duration is more than a given threshold.

Change-Id: I0e6a9f5746a22d2bfea2b21dd182489a34a4b528