SDK / exoplayer

EventTime contains information about when an event happened and where
it belongs to. Both places can be fully described using timeline, window
index, media period id and position.

Right now, only the information for where the event belongs to is fully
contained in EventTime, whereas the time when the event happened only has
the position, and none of the other information (timeline, window, period).

This change adds the missing information, so that the EventTime can easily
be used without having access to the Player. This also ensures Event
metadata is self-contained and can be stored and reused later.

issue:#7332
PiperOrigin-RevId: 311727004