AudioCodingModuleImpl is the only implementation of the
AudioCodingModule interface (except for test mocks). So it's a good
fit to put it in an anonymous namespace in the interface's .cc file,
to ensure that no one except AudioCodingModule::Create ever references
it.
Except for moving code, this CL introduces two other small changes:
* It cleans up the set of #includes in audio_coding_module.cc.
Specifically, I removed #includes that were already present in
audio_coding_module.h, and did not bring along any #includes from
audio_coding_module_impl.h and .cc except those that were
necessary to get it to compile.
* It moves AudioCodingModuleImpl from the webrtc::acm2 to the
webrtc::<anonymous> namespace. This means I had to qualify a few
things it references with acm2::.
Review-Url: https://codereview.webrtc.org/2069723003
Cr-Commit-Position: refs/heads/master@{#13191}
c13ded54ca