Add UserSwitchObserver.onBeforeUserSwitching and make SysUI use it.

During a user switch, allow SystemUI to update its source of truth
for current user before WallpaperManagerService, to put wallpaper
colors changed events in order. Otherwise, ThemeOverlayController
receives onColorsChanged call before UserTrackerImpl updates current
user id and thinks colors are changed for non-current user.

Bug: 274925897
Test: atest UserTrackerImplTest
Test: atest UserControllerTest
Change-Id: Ib5e467a18d75d633b2ebaecbaf00fb65f8be6c77