Make running auto-off timer visible in UI

[freddair]

Let's expose the countdown timer visible in the UI beneath each switch auto-off setting.

Let's also disable the update button with a note if auto-off is running (thanks @timoschilling ), because device won't accept update if it's in "doing something" mode, including auto-off timer (thanks @rojer ).

https://github.com/mongoose-os-apps/shelly-homekit/issues/437

[rojer]

SGTM

[rojer]

real solution for the auto-off timer is to persist its state and restore on reboot so it continues running, but for now disabling update will do.

[andyblac]

@ZeevoX or @timoschilling could one of you guys take this on ?

[andyblac]

and maybe this could be done at the same time ? https://github.com/mongoose-os-apps/shelly-homekit/issues/209

[timoschilling]

I can take a look on this issue, but #209 sound like a different feature.