AudioService: fix death linking for external audio policy

When using an external focus policy, link to death of focus
  requester (death handler was created, but not activated).
Fix error code path with external focus policy: on error at
  request, do not use the legacy code path, abort instead.

Bug: 128541285
Test: see bug
Change-Id: I4b118308eb0b5b2220eaeb822c2302b941119f9b