encoder: fix audio only initialization

MediaRecorder only calls the MediaEncoder with an AVFrame*, not with our
VideoFrame/AudioFrame wrappers, so the audio stream was never
initialized.

Gitlab: #95
Change-Id: Ie9c85eb9334444f2b58a4355e8f6db9d2a14610f
Reviewed-by: Sébastien Blin <sebastien.blin@savoirfairelinux.com>