audiobuffer: add conversion to AVFrame

Moves the AudioBuffer to AVFrame conversion logic outside of the
MediaEncoder, so the latter only deals with AVFrame.

Does not copy the buffer, the resulting AVFrame will simply point to the
existing buffer.

This is the first in a series of patches attempting to rid Ring of its
RingBuffer/RingBufferPool system.

Change-Id: I17c9fcdb96dc945c3eeace647303339d558bcf38
Reviewed-by: Adrien Béraud <adrien.beraud@savoirfairelinux.com>