Use type as a proxy to isLive in dash_parser

[baconz]

Static assets do not necessarily specify a mediaPresentationDuration, so we cannot use presentationDuration == null as a proxy to isLive.

Confirmed that this fixes #463

[joeyparrish]

In my opinion, we have too many definitions of isLive now. We used to have one source of this, in PresentationTimeline.

We need to step back and reassess what we're trying to represent and how. I suspect that the term "live" has become overloaded and there are multiple concepts that need to be represented separately.

[baconz]

I think that there are three questions which at one point or another have been associated with "isLive":

1. Is there a known duration?
2. Are all of the segments currently available? What is the availability end?
3. Is the back-buffer dynamic? What is the segment availability duration?

In my view, "isLive" should always refer to (2) on that list, and I don't think there are any places in the code where we're conflating it with the others (anymore?). As far as I can tell, the best proxy we're going to get for (2) from the manifest is type=dynamic, so I think this PR is still valid.

The reason some of this logic needed to be pulled out of the PresentationTimeline was that we need it when populating the SegmentIndex, before the timeline is fully initialized. It probably makes sense to add an instance variable to PresentationTimeline that is taken directly from context.isLive and use that in PresentationTimeline.isLive to avoid any confusion.

Right now we signal (1) and (3) with timeline.getDuration() == Number.POSITIVE_INFINITY and timeline.getSegmentAvailabilityDuration() == Number.POSITIVE_INFINITY respectively. We could throw those into their own methods on timeline for clarity.

Let me know your thoughts, and I can submit a revised PR.

[joeyparrish]

As discussed via email, we are pursuing a different fix. Closing this PR for now.