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

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

BUG=webrtc:5801

Review-Url: https://codereview.webrtc.org/2339953002
Cr-Original-Commit-Position: refs/heads/master@{#14274}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 1e4d8b574cde64d93b98d89c7b817fb93185a307
6 files changed