audiohal: Enforce flush of hwbinder commands queue after cleaning client refs

In binderized mode, the exact time of destroying the server object
is not defined. Enforce destruction by flusing hwbinder commands
queue after releasing the last client reference.

Bug: 35394629
Test: capture trace while following repro steps, verify that server-side object
      gets destroyed

Change-Id: Id2756cab0f36ed4eaf290d3f5a1c750c9c75b1d3