SDK / exoplayer

Using chunkless preparation greatly improves start up time if the master
playlist declares CODECS for the renditions. Hence, we turn this on
by default as it benefits most well-defined HLS master playlists.

The only known reason why developers may want to turn this feature off is
when the renditions contain muxed closed-caption tracks that are not
declared in the master playlist. So this change also updates the documentation
and RELEASENOTES to point out this caveat.

PiperOrigin-RevId: 413950036