SDK / exoplayer

The folder type has a mix of information about the item. It shows
whether the item is browsable (type != FOLDER_TYPE_NONE) and
which Bluetooth folder type to set for legacy session information.

It's a lot clearer to split this into a boolean isBrowsable and
use the existing mediaType to map back to the bluetooth folder type
where required.

folderType is not marked as deprecated yet as this would be an API
change, which will be done later.

PiperOrigin-RevId: 493544589
(cherry picked from commit 9d059352)