Backport !691 “gs-shell: Overview page opens empty with "Automatic Updates" off” to gnome-40

When the "Automatic Updates" are off, the shell window switches to the overview page immediately. Since some recent change the preselected page is the Overview page, thus the switch to it doesn't call reload(), thus it is left empty. To adapt to the new behavior call reload() also when the Overview page had been already selected. It's okay, because it's only after the initial start.

Non-trivial cherry-pick of 80fc7459 to gnome-40.

Fixes: #1070 (closed)