Revert "[media] Move Metadata into fuchsia.media."

This reverts commit e9797233a302400cecd50add09157d76ad8e289f.

Reason for revert: <INSERT REASONING HERE>

Original change's description:
> [media] Move Metadata into fuchsia.media.
> 
> This type can be shared by fuchsia.mediaplayer and
> fuchsia.media.session.
> 
> TEST: CQ
> Change-Id: Iea7d28b19e8b71565bb01366acf26bbc813f2c1b

TBR=dalesat@google.com,mpuryear@google.com,turnage@google.com

Change-Id: I2f3955d85a2f0df38db3df0da00723f5783c73bb
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
16 files changed