The KeySource now only handles fetching the keys and loading any PSSH
info from the license; it will not handle generating new PSSH info
based on the config.
This will allow the PSSH generation to access to the full
EncryptionConfig so we can add additional options to it.
Issue #756
Change-Id: Ia67387aa3d5ec0d723b7f5f21fc517f64c840393