SDK / exoplayer

DataSpec.FLAG_ALLOW_CACHE_FRAGMENTATION is added to indicate to the
cache when fragmentation is allowed. This flag is set for progressive
requests only.

To avoid breaking changes, CacheDataSink defaults to ignoring the flag
(and enabling fragmentation) for now. Respecting the flag can be
enabled manually. DownloaderConstructorHelper enables respecting of
the flag.

Issue: #4253
PiperOrigin-RevId: 229176835