Allows streamlining the way we handle unsupported DRM. In general, either:
* The device does not support the CDM. For example, phones don't support
PlayReady.
* The device does not support the protection scheme. For example, API < 25
devices do not support AES-CBC.
Currently we handle both cases differently. A dummy ExoMediaDrm will allow
the existence of dummy DefaultDrmSessionManagers.
PiperOrigin-RevId: 269315687
