audioflinger: enable volume listener on direct outputs

Always allow effects with no process function on any output
thread type as there is no impact on performance and no risk of
the audio properties not being supported by the effect.

Bug: 63015903
Test: place a Skype call over SCO and verify the volume listener
can be created.

Change-Id: I99db7802cec88a618808e500b74f4c78165199fe