We currently have a currentMediaPeriodId and an activeSessionId that are more
or less tracking the same thing unless the current media period isn't "active" yet.
Simplify this logic by using a single currentSessionId field and the respective
isActive flag of this session. Also move all session creation and activation code in
the same method to make it easier to reason about the code.
This change also fixes a subtle bug where events after a seek to a new window
are not ignored as they should.
PiperOrigin-RevId: 289432181
145754618d