The live fallback ladder, retry latch, timeline suspend/resume, and
exit-on-resume flags were poked directly from the error handler, the
play-intent path, the playing-state listener, and lifecycle handling.
Give the live part named operations (_beginLiveLadderRetry,
_retryLiveStreamForPlayIntent, _resetLiveLadderOnPlaybackRestart,
_stopLiveSessionForTvBackground, _consumeLiveExitOnResume) and move
the timeline suspend/resume helpers over from lifecycle, so
LiveTvSessionState is mutated only by live_tv.dart and the initial
tune in the start composition root.
edde746
5842d30ae8