Separate sniffing from session initialization

This avoid lengthy/duplicate sniffing for drm plugins when a decrypt session is opened

o The change is backward compatibile in that no update is required
  for existing drm plug-ins if they do not plan to provide separate
  sniffer/extractor

related-to-bug: 5725548

Change-Id: I7fc4caf82d77472da4e2bc7b5d31060fb54fd84c