audio policy: fix call volume upon speaker to USB transition

Fix voice volume handling in checkAndSetVolume() to only update
voice volume when changing volume on the primary output.
By convention, call routing and volume is managed by the primary output
and as other outputs may use a different device than the device selected
for calls, a wrong voice volume can be computed and overwrite a correct
volume set previously.

Bug: 195907034
Test: repro steps in the bug
Change-Id: Iafecb4df159db84addc3d802cdd2775cc696e07e