Singularization of duplicated multi user strings.

Many multi-user strings were duplicated between SettingsLib, Settings,
System UI and Car projects. This CL removes those duplicates.

List of singularized strings: "Guest", "Remove guest", "Add user",
"Remove guest?", "Too many incorrect attempts...."

Bug: 191386817
Test: NA
Change-Id: I8d9629a1a9628cab9f049df84f706bd04ee75cc2