Create Playhead Interface

We need to have a playhead for media source and src=, however the
current playhead implementation is tied very tightly to manifests.
Rather than working to refactor playhead, this change defines an
interface and makes the old playhead the Media Source implementation of
the interface.

In a later change the src= version will be introduced to allow playhead
functionality when we don't have a manifest.

Issue shaka-project#816
Issue shaka-project#997

Change-Id: Ie74e50839c79c3cd3acf14000849e3d5fa71a42f