Keep heart rate tracking setting after reboot

[Andersama]

When the watch boots check the user's last known setting.

See: #2364

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[Andersama]

I've tested this, it appears to work, no issues so far.

Update: A small issue, the settings aren't saved when the button is enabled / disabled (which is vaguely what I hoped would happen), they're saved whenever the system decides to write the settings. The hacky approach I've found is to toggle any of the options in the settings area.