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", "New user", "Add new user?",
"When you add a new user....", "Add user", "Remove guest?",
"Too many incorrect attempts...."

Bug: 191386817
Test: NA
Change-Id: I3689cfdbc3e0fc2f6a773941d044d8b32a762e88