Add support for MultiKey DRM
Created by: epiclabsDASH
This PR adds support for MultiKey DRM (Fix #1595 (closed)). Tested sucessfully with Clear Key, Widevine and PlayReady.
When initializing a stream, Stream class uses ProtectionController to initialize all possible adaptations of the current period. Later on, in the session creation process and once each keysystem is selected and initData available, we will create just the sessions with unique combinations of initData/keysystem.