Before this change constructors of `TimelineWindowDefinition` used
`windowOffsetInFirstPeriodUs` for both, `positionInFirstPeriodUs`
and `startTimeUs` of the exposed `Timeline.Window`.
Adding `windowStartTimeUs` differentiates these two properties which
allows using `FakeTimeline` for unit tests that emulate live streams
with a forward moving live window in the period like `HlsMediaSource`
constructs a `SinglePeriodTimeline` for live.
A `TimelineWindowDefinition.Builder` is added for API usability and
sanitation.
PiperOrigin-RevId: 730892809
cecee3bdda