feat: add "on backgrounded" function

[EvanHahn]

MapeoManager#onBackgrounded() should be called when the app goes into the background. It will do its best to gracefully shut down sync.

Closes #576.

[EvanHahn]

Simplified this a bunch (I think).

When the app is in the background and sync is complete, cores will be un-replicated. There is no longer an auto-stop timeout, but it should be easy to add in the future.

There are now two booleans: wantsToSyncData and isBackgrounded.

• start() enables wantsToSyncData.
• stop() disables wantsToSyncData.
• background() enables isBackgrounded.
• foreground() disables isBackgrounded.

After these operations and when the state is updated, #updateState() is called. This figures out the desired SyncEnabledState and applies it.

[EvanHahn]

Thanks for all the review here. Hopefully this is the last round!