AcmReceiver: Look up last decoder in NetEq's table of decoders

AcmReceiver::decoders_ is now one step closer to being unused.

(This is a re-land of https://codereview.webrtc.org/2339953002.)

BUG=webrtc:5801

Review-Url: https://codereview.webrtc.org/2354453003
Cr-Original-Commit-Position: refs/heads/master@{#14303}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 6f0f616b53c0a7b95f7616fdcafe67ec595b2b33
6 files changed