Inherit RefBase virtually in AudioDeviceCallback

In cases where we have diamond inheritance from RefBase, such as
implementing two different callback interfaces, we need to share
the reference count. Two seperate reference counts lead to
difficult to detect lifetime issues.

Bug: 207034551
Test: Compiles
Change-Id: I4c6e0bca2fdf65eaed5029b3ec8a7b6a39b3f15f