media: refine kMetadataBufferTypes

- Fix kMetadataBufferTypeANWBuffer, so that it only relies on
system/core structures. This resolves potential linker issues if
the framework changes.
- Add fence to be passed with kMetadataBufferTypeANWBuffer.
- Add utility structures to access the metadata buffer layouts.

Related-bug: 19614055
Bug: 13222807
Bug: 12386081
Change-Id: I608958fd394b08cc3396bcbd5cdd350b87bc66f3