DASH periods don't have to start at the beginning of a segment. In
these cases, they should report an initial discontinuity to let the
player know it needs to expect preroll data (e.g. to flush renderers)
This information is only available in the ChunkSampleStream after
loading the initialization data, so we need to check the sample
streams and tell them to only report discontinuities at the very
beginning of playback. All other position resets are triggered by
the player itself and don't need this method.
Issue: androidx/media#1440
PiperOrigin-RevId: 668831563
e8664dbc8e