audio: omx encoder isn't expected for raw recording

Raw recording doesn't require any encoder, so no OMX encoder will be
created and audio source will be directly used as media source.

UNKNOWN_ERROR error will be threwn if call AudioSource::start()
continuously, so we should avoid omx encoder operations if it's not
created for raw recording.

Change-Id: Ib38c4e42b7667e9bf882cddb15422d0dd598b60e
CRs-Fixed: 752419