In preparation for adding support for ads in playlists:
- Make releasing a no-op if the instance was already released
- Remove null checks on non-null `adDisplayContainer` and `adsLoader`
- Move initializing the ads manager into a private method as it will need to be
called from two places soon.
- Misc other cleanup.
Issue: #3750
PiperOrigin-RevId: 341021493
