Spec: https://storage.googleapis.com/wvdocs/Widevine_DRM_Android_Using_Key_Rotation.pdf 1. Implement multisession to support drm key rotation 2. Put MediaDrmEventListener back to manager since this is a per mediaDrm thing. 3. It seems diffrenciate between single/multi session is unnecessary. ------------- Created by MOE: https://github.com/google/moe MOE_MIGRATED_REVID=167602965
