This moves the position estimation and plausibility checks inside
the class.
Mostly a no-op change, except that the plausible checks now use
the estimated timestamp from getTimestamp and the playback head
position to make them actually comparable. Before, we were comparing
the last snapshot states which may not necessarily be close together.
Given the large error threshold of 5 seconds, this shouldn't make
a practical difference though, just avoids noise and confusion.
PiperOrigin-RevId: 754035464
ffb8d3ca66