Fix AudioManager.forceVolumeControlStream()

AudioManager.forceVolumeControlStream() is used by VolumePanel to temporarily force the
stream type which volume is controlled by volume keys.

Current implementation is not working if the VolumePanel is not executed by the same process
as the one receiving the volume key events.

Issue 6302421.

Change-Id: I2700587a027ffb962429b42083312cd92fe79215