endSessionDialog: default to not installing updates on low battery

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 #2717 (closed)