SystemUI: implement properly saving tile locations

Use new persisted key which does not use the tiles' ID (which changes
usually across reboots with every tile). When the Custom tile service
comes up, it will update the placeholder tiles created with their real
values and the order is preserved properly.

Ticket: CYNGNOS-2530

Change-Id: I5e75c739553969f1d92c249f521a476259fe0cd6
Signed-off-by: Roman Birg <roman@cyngn.com>