base: Fix notification sounds for wifi only devices

The linking of voice & notification doesn't take into account the
fact that a device may not actually have voice capabilities.  Whenever
checking whether or not to link notification and ring volumes, also
verify that the device really is voice capable.

Change-Id: Iae49302d3b5934d36b182e289e4975f5dd85b834
(cherry picked from commit 2870d05e)