media_recorder: set encoderStream to filter's output params

In setupAudioOutput we initialize an audio filter using the
encoder stream but this filter is left unused and the
initial encoder stream is returned instead.

Set encoderStream to filter's out params.

Change-Id: Ibe0e51946a630699f4d6528692c9930462cf18b6
Reviewed-by: Philippe Gorley <philippe.gorley@savoirfairelinux.com>