We currently force discontinuities for all clipped content periods
that don't start from zero. This shouldn't be done for server-side
ad insertion streams where we are guaranteed to get a continuous
underlying stream.
To enable the right decision, we need to add a flag to
MediaPeriodInfo for isPrecededByTransitionFromSameStream, which
mirrors the existing isFollowedByTransitionToSameStream.
The problem is currently not visible due to a bug in
ClippingMediaPeriod that automatically ignores most discontinuities
that don't match the start of the clip.
PiperOrigin-RevId: 713315398
7d4ddfbb91