Broke out CNG from AudioDecoder as they didn't really share an interface.
Converted the CNG code to C++, to make initialization and resource handling easier. This includes several changes to the behavior, favoring RTC_CHECKs over returning error codes.
Review URL: https://codereview.webrtc.org/1868143002
Cr-Commit-Position: refs/heads/master@{#12491}