Fix NPE while running UserSwitcherController

The mRootView in UserSwitcherController is not initialized while running
the test, fulfill the dependencies and initialize it before testing the
function.

Bug: 206333323
Test: atest com.android.systemui.statusbar.policy.UserSwitcherControllerTest
Change-Id: I807ac2d1279b4f48c94e5d9ccd51f3a43d5884c5