Adding in API to allow for DVR information used to drive a player UI

Added new class called videoElementExt with and API to help with DVR window. We could add other video element API to this as well in the future. Added metric to hold the information for DVR streams. Wanted the API to be a singleton and the metric VO to just hold the values for the API to use.
Mikhail suggested that this should either replace DashManifestExtensions.getIsDVR or should call DashManifestExtensions.getIsDVR