Refine Bluetooth Metadata API

- Modify MetadataListener to as an interface and rename
  it to OnMetadataChangedListener
- Fix typo UNTHETHERED -> UNTETHERED
- Add NonNull annotation for metadata API parameters
- Re-design metadata unregister API
- Change metadata type to byte array

Bug: 124448651
Bug: 126701203
Bug: 126699213
Test: build pass

Change-Id: I79460071c7693f648e92cf849738c24f8bc269d9
Merged-In: I79460071c7693f648e92cf849738c24f8bc269d9