In multi-period live streams, we can't use the content timeline if we want to
lookup a period from the public timeline by index or uid because it may be
that the content timeline has already been refreshed in the
`ImaServerSideAdInsertionMediaSource` but hasn't yet arrived in `ExoPlayerImpl`.
This change is taking the current position that needs to be reported
to the SDK every 200ms is not relying on the content timeline.
PiperOrigin-RevId: 520328126
