Properly invalidate the show/hide system apps menu.

The old code only invalidated the options menu when it sees system
apps for the first time.  Now that we can change the definition of
what is a system app, we should also invalidate the menu when there
are no system apps when there used to be some.

Fixes: 129477843
Test: Open dashboard, Advanced settings, toggle global/per-app switch,
go back, refresh, see correct menu items.

Change-Id: Ic20e040b57d51d94fdd3ec7e1e2f3ed50cee9668