Fix NPE in UserSettingsTest

Also, introduce a check for ACTION_USER_GUEST_ADD being logged correctly

Test: m RunSettingsRoboTests ROBOTEST_FILTER="com.android.settings.users.UserSettingsTest"
Bug: 183920589
Change-Id: Ie4fdf4b3429a9abec64120a38a13f2d8a3fa121a
1 file changed