Multi period content support for AdsMediaSource

[kotucz]

Multi period DASH content ads support inspired by #1642 (comment)
At Timeline level, the original AdPlaybackState is divided and adjusted for each of the periods.

Removing single period assertions from AdsMediaSource.
Providing MultiPeriodAdTimeline as an alternative to current SinglePeriodAdTimeline.
From input AdPlaybackState, new modified AdPlaybackState is created - one for each period:

• ad group time is offset relative to period start time
• ad groups after period end time are marked as skipped
• post-roll ad group is kept only for last period
• ad group count and indices are kept constant

[google-cla]

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[icbaker]

Please sign the CLA, then we can take a look at this PR

[kotucz]

Company CLA should be used. It is in progress