UserCredentialsSettings: that fresh new blank look

Less obviously easy than it looks because we want to make it a
SettingsPreferenceFragment for consistency with the other blank
fragment implementations.

Fix: 30878596
Test: make RunSettingsRoboTests
Test: # open up Settings and have a look, uninstall cert, reinstall cert
Change-Id: Ifbe3132475c3104d76589a50dec3f436b9548585