If we prepare a deferred media period before the actual timeline is available,
we either prepare with position zero (= the default) or with a non-zero
initial seek position.
So far, the zero (default) position got replaced by the actual default position
(including any potential non-zero window offset) when the timeline became known.
However, a non-zero initial seek position was not corrected by the non-zero
window offset. This is fixed by this change.
Related to that, we always assumed that the deferred media period will the
first period in the actual timeline. This is not true if we prepare with an
offset (either because of an initial seek position or because of a default
window position). So, we also determine the actual first period when the
timeline becomes known.
Issue:#4873
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=215213030
46731cdd1d