Make Launcher activity restart on density change

- Launcher used to restart when density changes before http://ag/14234761, which added smallestScreenSize and density in configChange
- http://ag/14234761 aims to avoid restart when switching display in split dispalys, for which only size changes, therefore density is unnecessary in configChange
- Keeping density in configChange make Launcher acitvity to stay after changing display size, making all buttons stays in old size in previous denstiy, which is a bug

Bug: 204157738
Test: manual
Change-Id: Ib85326078c67c35d90dc5e9ae5c20eeb5286a610