[res] Properly exclude system overlays when requested

Getting non-system locales / configurations was broken for the
case when there are no non-system overlays present - it used to
assume all overlays are non-system instead of the opposite. This
was causing the wrong list of locales returned in the API

Bug: 421654198
Test: atest libandroidfw_tests
Flag: EXEMPT minor bugfix
Change-Id: I612a8526ae09175ea337faa2d37ea47bfbc08aa9