Add QuickstepWidgetHolder for widget handling

Fix: 235358918
Test: Manual
1. Rebooted the device and verified that widgets are still updating properly
2. Changed the theme from dark to daylight, then from daylight back to dark and verified that widgets are working
3. Kept the device on for several days and verified that widgets are still updating
4. Turn on auto-rotate for the launcher, open any app then exit, verified that widgets will not vanish and reappear
5. Add & remove widgets from the screen, added widgets are still updating

Change-Id: I98ee902f7d16b47bd77626201a4fefc897ba17a0