audioflinger: Fix RefBase "exception" issue from FastThread

Dummy NBLog::Writer in FastThread must not be stored by value
because it's a RefBase descendant. Storing it by value causes
a diagnostic output from ~RefBase because no strong
references to it ever exist. In future this will cause a crash.

Test: verify there are no "RefBase: Explicit destruction" messages
      in logcat
Change-Id: I95716ec8e29fdd9485ef64a3334cef49bf29bc96