Some events may arrive after the playlist is cleared (e.g. load
cancellation). In this case, the DefaultPlaybackSessionManager may
create a new session for the already removed item.
We already have checks in place that ignore events with old
windowSequenceNumbers, but these checks only work if the current
session is set (i.e. the playlist is non-empty). The fix is to add
the same check for empty playlists by keeping note of the last
removed window sequence number.
PiperOrigin-RevId: 541870812
(cherry picked from commit 656e0723e51e1bc38d558e77fa2cb0cc17e37054)
