In order to expose the live window, it is necessary (unlike before) to refresh
the live playlists being played periodically so as to know where the user can
seek to. For this, the HlsPlaylistTracker is added, which is basically a map
from HlsUrl's to playlist. One of the playlists involved in the playback will
be chosen to define the live window. The playlist tracker it periodically.
The rest of the playilst will be loaded lazily. N.B: This means that for VOD,
playlists are not refreshed at all. There are three important features missing
in this CL(that will be added in later CLs):
* Blacklisting HlsUrls that point to resources that return 4xx response codes.
As per [Internal: b/18948961].
* Allow loaded chunks to feed timestamps back to the tracker, to fix any
drifting in live playlists.
* Dinamically choose the HlsUrl that points to the playlist that defines the
live window.
Other features:
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The tracker can also be used for keeping track of discontinuities. In the case
of single variant playlists, this is particularly useful. Might also work if
there is a that the live playlists are aligned (but this is more like working
around the issue, than actually solving it). For this, see [Internal: b/32166568]
and [Internal: b/28985320].
Issue:#87
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