SystemUI: fix blank qs tile issues

The blank tiles were an artifact of precaching custom tiles. We need to
not show them or count them if they don't have an actual tile attached
to them yet.

Also, allow disabled tiles to be edited.

Ticket: CYNGNOS-2530

Change-Id: Ie6e0e6e18435eb53e9b419a05d780bdd1ee808b7
Signed-off-by: Roman Birg <roman@cyngn.com>