SDK / exoplayer

This change optimizes startup and track selection for HLS. Changes
in HlsChunkSource avoid unnecessary re-requests for media playlists.
Changes in HlsSampleSource optimize exit from the limbo state (i.e.
when endTrackSelection is first called).
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=118026962