SystemUI: remember deleted custom tiles

We previously had no mechanism of knowing which tiles the user had
manually removed, leading to the tiles returning when the publishing
application decided it was time to update.

Now, when the user removes a custom tile; set a flag and hide it in the
tile list. The removed tiles will be enumerated in the Add tile detail
view if the user decides they want it back.

Ticket: CYNGNOS-2530

Change-Id: Ia4fe74ef580af7139279a2fb78e240e872f8dbb0
Signed-off-by: Roman Birg <roman@cyngn.com>