Fallback activity for supervised user creation was creating a supervised user
without a name (null), now creates it with a default name.
Also put an explanation on the activity stating it's just a placeholder.
Bug: 215232217
Bug: 207495450
Test: croot && make RunSettingsRoboTests -j40 ROBOTEST_FILTER="com.android.settings.users.UserSettingsTest"
Change-Id: I8090fde38155817ac2b4f88271abe8a0ad1ce747
Create a simple fallback activity to be started if no other activity could be
resolved when Add supervised user option is tapped in Multi User settings.
Bug: 207495450
Test: croot && make RunSettingsRoboTests -j40 ROBOTEST_FILTER="com.android.settings.users.UserSettingsTest"
Change-Id: I850a2798567787c2494c75aa5fa451ddf373410d