audio policy: refactor getOutput() method

getOtuput() was long replaced by getOutputForAttr() for the purpose of
querying an appropriate HAL output stream for a given playback use case.
It is now only used internally by AudioSystem to find an existing output
corresponding to a given stream type in limited cases.

This CL simplifies getOutput() by removing unused arguments and just
returning an output from already opened outputs.
This is in preparation of limiting access to audio policy binder calls
to what is strictly needed by apps.

Test: test main playback, capture and call use cases.
Change-Id: I137c5526aa7e82303b43948326c77ec798f432f5