dart client: Ensure Monitor is always running if it is expected to be running

[gkc]

Describe the bug

An issue occurred where monitor died in an emulator instance of an app, and monitor did not restart itself.

Defensive code (most likely a cron in the NotificationService) required in order to ensure that the monitor is always kept alive unless it is in the Stopped state.

This bug needs to be fixed immediately, but see also #1032 for an enhancement to the broader issue here, of component sanity when resuming from background.

Steps to reproduce

So far, the issue is not reproducible on demand by @gkc. However gkc has seen the problem occur very occasionally when for example sshnpd is running on computer and the computer resumes from sleep. There's a race condition somewhere, which isn't surprising as the current code is a bit over-complicated and silly. (What clown wrote that code? Oh, it was me.)

Expected behavior

Monitor should always be running if it is in Started state.

[gkc]

No progress last sprint due to higher priority work

[gkc]

Some progress in PR67, might complete in PR68 if the other more important stuff gets done

[gkc]

Moving to current sprint as this is a significant issue, although it happens very rarely

[gkc]

Implemented and manually tested during PR69 but need to add multiple additional unit tests before raising the PR

[gkc]

Got back to this during PR72, added some more unit tests which exposed another problem, working on resolving that problem now. Expect to complete the PR early in PR73

[gkc]

Finished during PR74, will create PR during PR75

[gkc]

Hoping to make time to complete this in PR80 if I can find a day