Add metadata keys for opaque CSD

This provides a generic 'passthrough' of codec specific data
from extractor to codec, without having to add conversion to/from
a format-specific key in Utils.cpp
Remove unused kKeyFlacMetadata, and unneeded vorbis conversions in Utils.cpp

Test: build, CTS

Change-Id: I177090c0b0cb5174031c68ec7ec2abf6fa15c360