Make sure activity for adding a supervised user is opened full screen.

Add supervised user activity only covers the right hand side of the
screen in large screen devices. This CL makes sure it is always
started from Settings with correct flags to be shown full-screen.

Bug: 214401383
Bug: 205101183
Bug: 199868785
Test: croot && make RunSettingsRoboTests -j40 ROBOTEST_FILTER="com.android.settings.users.UserSettingsTest"
Change-Id: If81e0cce91a5295eb2b93b12f68b70fd1240c953