SDK / exoplayer

This is a candidate fix for #8906. As mentioned in that issue,
negative positions within windows might be (kind of) valid in
live streaming scenarios, where the window starts at some
non-zero position within the period. However, negative positions
within periods are definitely not valid. Neither are positions
that exceed the period duration.

There was already logic in ExoPlayerImplInternal to prevent a
resolved seek position from exceeding the period duration. This
fix adds the equivalent constraint for the start of the period.
It also moves the application of the constraints into Timeline.
This has the advantage that the constraints are applied as part
of state masking in ExoPlayerImpl.seekTo, removing any UI flicker
where the invalid seek position is temporarily visible.

Issue: #8906
PiperOrigin-RevId: 395917413