Codec Kit: Move Managers to private headers

* Those classes are not ready for public consumption. Ideally,
I'd add a well designed BCodecRoster wrapping them. This is part
of the general cleanup I am doing to get the code in a good state
before going to finalize the design.
* I don't plan to reintroduce BMediaFile in the media2 API, and
I'd like to remove any (explicit) usage of entry_refs and things
like that.
* I plan to introduce BMetaData and BMediaFormat which is going
to be different than what we do now.
* We need to explicitly use the mime type when it's available and
it is another design consideration when CodecRoster will be introduced.

src/kits/codec/Jamfile

@@ -1,6 +1,6 @@
 SubDir HAIKU_TOP src kits codec ;
 
-UsePrivateHeaders app media shared ;
+UsePrivateHeaders app codec media shared ;
 
 if $(CHECK_MALLOC) {
 	SubDirC++Flags -D_NO_INLINE_ASM -fcheck-memory-usage ;