Include the returned IAudio{Track,Record} in response parcelable

In preparation to AIDLizing IAudioFlinger, having two return values is
inconvenient. It is also not consistent with most other methods, which
mostly return status_t. This change puts the returned
IAudio{Track,Record} in the response parcelable instead of having them
as separate return values.

Test: Audio-related CTS tests from CtsMediaTestCases
Change-Id: I97cbec21ca936b4e00e5caa06760ce00922739e8