Don't use a pointer for the A2DP Sink static mutex

The A2DP Sink cleanup might be called even if A2DP Sink init wasn't called
before, hence the g_mutex local pointer might be null.
We cannot check whether the pointer is null, because the A2DP Sink init
and cleanup operations are not executed on the same thread.

Bug: 68255348
Test: Manual
Change-Id: I9881ada0c4d01192352621ca5fe024f0e8b0ef76