Keep list of available formats when using HLS chunkful preparation.

[tonihei]

[REQUIRED] Use case description

When using HLS chunkful preparation or when using a media playlist directly, we need to load at least one of the segments to determine the available formats. However, we do this for every created MediaPeriod, which is wasteful because the formats are not supposed to change if the preparation completed once.

Proposed solution

We should keep the list of tracks in the HlsMediaSource and use it for all subsequent MediaPeriods without loading a segment again. This also prevents problems like #6314 from occurring in the first place.

Alternatives considered

n/a