Extract UserShortcutType functions from ToggleFeaturePreferenceFragment

* Simplify StringJoiner to '+' since it only have one case
* Simplify Collections usage to use removeIf() and findFirst() directly
* Change class name SharedPreferenceUtils to PreferredShortcuts

Bug: 158540780
Test: atest ToggleFeaturePreferenceFragmentTest
Test: atest PreferredShortcutsTest
Change-Id: I02f03ccba09b6d7edaa5c0c8223ab3561a5e976b