getKeysFromEntitlement
Unwrap the content keys carried by an ENTITLED_KEY PSSH.
Entitlement licensing splits delivery in two: a first license carries ENTITLEMENT keys, and a later ENTITLED_KEY PSSH carries content keys each wrapped with one of them. Each WidevinePsshData.EntitledKey names its wrapping key by entitlement_key_id and supplies the AES-CBC iv.
The session must already have parsed a license containing the matching ENTITLEMENT keys; getKeys returns those, this returns the content keys they unlock.
Return
the unwrapped content keys, which are not added to the session
Parameters
a PSSH whose CENC header is of type ENTITLED_KEY
Throws
if the session id is invalid
if the session holds no ENTITLEMENT keys
if the PSSH carries no entitled keys
if an entitled key is missing a required field
if no entitlement key matches an entitled key