SDK / exoplayer

In many cases it doesn't matter for the test itself how many windows
a timeline has, or even how the timeline of a MediaSource looks like.

And since we introduced the MediaItem-based APIs, single-window
Timelines are the only fully supported Timelines. Thus there is no
point in specifiying this explicitly.

Using these assumptions, we can remove some boilerplate when setting
up standard FakeTimelines or FakeMediaSources with a standard
FakeTimeline.

#exofixit

PiperOrigin-RevId: 344210395