"Remove" fails for keys of sets

The `remove` function does not work as expected for values that are Sets of Strings. This issue is due to the way that sets are stored using multiple keys.

Expected:
```
SecurePreferences.setValue(context, "some_key", setOf("one", "two", "three"))
SecurePreferences.removeValue(context, "key_for_set") // does nothing to remove the set!!
SecurePreferences.contains(context, "some_key") // returns true. Should return false.
```