SharefPref StringSet implementation

This is implementing #264. The code is based on #543, but i changed the serialization to XML, as the conversion has come to this.
Actually this Reflection method was already used for `SharedPreferences.Editor.apply()` before my PR, i just followed that. But you are right, i'll use the API level so we can get rid of reflection and the whole `SharedPreferencesCompat` class.