[feature request] Save the game when entering the "pause" screen

[MrFullBelly]

The game doesn't save when entering the pause screen, or when switching to another app. This is troubling because sometimes the OS (Android 10 here) will unload a background app and this will cause a loss of state. When the pause screen is activated manually or because of switching to background the game should auto-save at that point. Equally when the game is unloaded by the OS.

Loss of game state is rather more common in practice than would be hoped for. This seems like a simple fix, a couple calls to a `save` function added in the right places.