Activity state stuff is automatically handled by Unity AFAIK. The game handles being switched away from gracefully in whatever state the game is currently in. Nothing can prevent the game being nuked by memory reclamation though, that has nothing to do with "Activity state". To restart an in-progress run after the game has been nuked by the OS, we'd have to defensively save some sort of game state to disk and then, when the game is restarted, check that file to see if the game should be in a dungeon or not. Theoretically, this is possible, but it's a massive amount of work that would have delayed the game's release by 4-6 months.
Nothing is easy and all the decisions you make as a game developer are apparently wrong, according to random people on the internet. Fun job!