API changes per API review for contacts

* Improve documentation for REMOVE_DUPLICATE_ENTRIES
* Hide android.provider.ContactsContract.PinnedPositions, it will be
reworked and improved for L.
* Hide android.provider.ContactsContract.Preferences - we will rework
the various apps that are relying on this to use their own preferences
instead
* Remove inheritance of hidden interfaces in android.provider.Contacts

Bug: 15430304
Change-Id: I0f96e8a506083df21023f9b95655f0ce5244bdce