Move audio decoding to protocol layer

Previously ChromotingClient was responsible for creation of
AudioDecodeScheduler. Moved that logic to IceConnectionToHost because
WebrtcConnectionToHost will not need it. Also moved AudioDecodeScheduler
from remoting/client to remoting/protocol and simplified it.

BUG=638505

Review-Url: https://codereview.chromium.org/2384063004
Cr-Commit-Position: refs/heads/master@{#422671}
22 files changed