Handle volume events on master volume devices correctly

We weren't passing volume events to the master volume correctly on
devices that only use a master volume. This fix checks if the device
only has a master volume and adjusts the master volume instead of the
stream's volume if that's the case.

bug:18305790
Change-Id: Iec35e0a7dc59e6d73c9dfc88da324660bb15b1f3