Standardize usages of "/" separator in SettingsLib

This separator should be translated the same as the
preference_summary_default_combination used in the Settings app. We
reintroduce this string here in order to avoid a dependency from
SettingsLib to Settings app.

Bug: b/62354743
Test: NA - Refactor
Change-Id: I2ce12848b944417b0df5e6f629aabd49712ff634