extensionPrefs: Block notify::active signal handler while updating state

We disable and enable extensions inside the notify::active signal handler, but we shouldn't do that in case the change didn't come from the user but because something else changed the state of the extension.

This causes an issue when the extensionPrefs window is open and the session gets locked: The extensions are temporarily disabled by the shell, extensionPrefs updates its switches on the state change and adds those extensions to the disabled-extensions gsettings key inside the signal handler. Now when the session is unlocked again, the extensions won't be enabled again since they're forced-disabled.