Adds offline decryption key and uses it to decrypt content during offline playback
Jira: VEX-5938
https://jira.tenkasu.net/browse/VEX-5938
- Update to accept scheme for key required to play offline playback
- Uses provided scheme to intercept call from player and return the key
- Fixes player item observer removal pattern
### Reviews
- Major reviewer (domain expert): @armadilio3
