Add util to convert Metadata into MediaMetadataCompat

### Issue description
Currently it's hard to extract metadata information and using it in the `MediaSessionConnector`. To make it easier i would suggest to add a conversion util that converts `Metadata` into a `MediaMetadataCompat` which can already be converted into a `MediaDescriptionCompat`.

This way it could be used to updated metadata on notifications, lockscreen and cast.

### Reproduction steps
None

### Link to test content
None

### Version of ExoPlayer being used
2.9.0

### Device(s) and version(s) of Android being used
All

### A full bug report captured from the device
None