Commander: Don't spam battery tune messages

[potaito]

When battery is low or critical, commander will publish a tune message. Not once though, but one message per thread cycle. The tunes library does take care of this and ignores messages with the same tune, but it's still unnecessary. Especially now when we have repeated tunes.

Solution:

Use flags to make sure the message is only sent once.

Adding more flags to the commander is not optimal, that I understand. But at least it will document the issue for future refactoring.

Implications

With this PR, the low and critical battery tunes can be interrupted by other tunes, since it's not continuously spammed.

[LorenzMeier]

Given those tunes can be important, could we add a priority (maybe 1-2 levels in the tone driver) and it resumes the tune it had loaded before? Have you looked at how difficult this is?

[dagar]

I definitely think these tunes need to be repeated at some rate. We should also think about doing it as a basic state machine and stop carrying so much unnecessary state.

[potaito]

@LorenzMeier Tunes currently have no notion of priority. We did have some priorities hard-coded in the tunes library, but that was of course a dirty hack. I also experimented with what you suggest, which is to have non-repeated tunes interrupt repeated tunes only temporary. It worked technically, but in practice it would require some tuning to make it obvious to the user, that it's two distinct tunes being played. Like adding pauses in between for example.

@dagar I somewhat disagree with you for once. If it is necessary to repeatedly send a tune, that means that the tunes library needs to be reworked. I believe sending tunes repeatedly will make deterministic results in the future more difficult, especially when 3rd parties use custom tunes instead of px4 defaults. Some parties might not even want the battery warnings to be repeated at all, which should be respected. Thus I think all information should be encapsulated in the tune definition.

[potaito]

Maybe I'll pick this up again some other time