Asterisk already supported iLBC 30. This change adds iLBC 20. Now, Asterisk
defaults to iLBC 20 but falls back to iLBC 30, when the remote party requests
this.

ASTERISK-26218 #close
ASTERISK-26221 #close
Reported by: Aaron Meriwether

Change-Id: I07f523a3aa1338bb5217a1bf69c1eeb92adedffa