This change does basically three things in preparation of a follow
up CL that will insert ad breaks into multi-period live windows.
- Make sure that only `AdPlaybackState`s of multi-period streams
are passed to `ImaUtil.splitAdPlaybackStateForPeriods()` from
the media source.
- Enhance the splitting algorithm in ImaUtil to support live windows.
Roughly, this includes calculating the window start position in the
UNIX epoch context, handling the end period with an unknown duration
and taking the `adResumePosition` of an ad group into account that
affects the ad group start positions (`adGroup.timeUs`).
- Append a placeholder postroll ad group to the ad playback state of
each period in a live stream to allow immediate transition to an
ad that's inserted by an ad event from the SDK.
PiperOrigin-RevId: 516167958
0588068f19