audio policy: fix wrong ring volume after music stops.

In computeVolume() only apply the special rule boosting
ringtone and notification volumes when music is playing over A2DP
if the selected devices do not included the speaker: In this case, the
notification volume applied is the volume for speaker, not A2DP anyway.

Bug: 187481909
Test: repro steps in bug
Change-Id: I1b419e6681f771c25f61092edbb3acbf946b8ba6