Add data class PreferredShortcut to replace inner class UserShortcutType

* Add basic function equals(), hashcode() into data class
* Change flatternToString() to toString()
* Change constructor to fromString(flatternToString)

Bug: 158540780
Test: atest PreferredShortcutTest
Change-Id: I0ee46dd940d22ff9f168b95fe75d9cff2f0fddfb