AudioNetworkAdaptor is supposed to facilitate AudioEncoder to adapt to varying network conditions. This is the first of a sequence of CLs that are to add one implementation of AudioNetworkAdaptor. This CL illustrates the interfaces of the AudioNetworkAdaptor. BUG=webrtc:6303 Review-Url: https://codereview.webrtc.org/2308573002 Cr-Commit-Position: refs/heads/master@{#14115}