Introduce CodecManager and move code from AudioCodingModuleImpl

This change essentially divides AudioCodingModuleImpl into two parts:
one is the code related to managing codecs, now moved into CodecManager,
and the other is what remains in AudioCodingModuleImpl.

This change also removes AudioCodingModuleImpl::InitializeSender. The
function was essentially no-op, since it was always called immediately
after construction.

COAUTHOR=kwiberg@webrtc.org
BUG=4228
R=minyue@webrtc.org, tina.legrand@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/51469004

Cr-Commit-Position: refs/heads/master@{#8893}

webrtc/modules/audio_coding/main/test/EncodeDecodeTest.cc

@@ -53,7 +53,6 @@ Sender::Sender()
 
 void Sender::Setup(AudioCodingModule *acm, RTPStream *rtpStream,
                    std::string in_file_name, int sample_rate, int channels) {
-  acm->InitializeSender();
   struct CodecInst sendCodec;
   int noOfCodecs = acm->NumberOfCodecs();
   int codecNo;