channel: write to a stream on multi-frame writes

If a frame handling routine returns a list of frames (vs. a single frame)
those frames are never passed to a tech's write_stream handler even if one is
available. For instance, if a codec translation occurred and that codec
returned multiple frames then those particular frames were always only sent
to the tech's "write" handler. If that tech (pjsip for example) was stream
capable then those frames were essentially ignored. Thus resulting in bad
audio.

This patch makes it so the "write_stream" handler is appropriately called
for all cases, and for all frames if available.

ASTERISK-28795 #close

Change-Id: I868faea0b73a07ed5a32c2b05bb9cf4b586f739d