Add user switcher related strings to SettingsLib.

- Move guest related strings from sysui to settingslib
- Add a new string to settingslib

Test: Build, flash and check if the texts are appearing properly in keyguard and quicksettings user switcher.
Bug: 142798722
Change-Id: I5b3e13581efa274ce8ed899b8a949e5ef69c173c