Moved codec-specific audio packet splitting into decoders.

There's still some code run specifically for Opus w/ FEC. It will be
addressed in a separate CL.

BUG=webrtc:5805

Review-Url: https://codereview.webrtc.org/2326003002
Cr-Original-Commit-Position: refs/heads/master@{#14319}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 0d526d558b0f7b677c43f83e64e67ad65a9639c0
26 files changed