If the user's battery power is low, we should not check the checkbox to
install updates by default. Rationale: if the user's battery is not low,
it's very unlikely to run out during a normal system upgrade. Low
battery is defined as any level below 30%, matching our battery status
indicator.
We'll also change the battery warning to only display when battery is
actually low. However, we will still always warn on battery for full
system upgrades, since these are expected to take a long time.
Future improvement: it would be nice to make the checkbox insensitive
when on low power. However, I don't think we currently have a proper
style for insensitive checkboxes. I was unable to make it look good.
Lastly, note that I did not test this on a laptop. I tested this by
mocking the return values of _isDischargingBattery() and
_isBatteryLow().
Fixes https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/2717
6d38bc69ca