Set default name when creating a supervised user in fallback activity.

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