Fix new contact not being loaded in QuickContact

When ContactLoader is loading new contact, the reference to old contact
should be set to null so that ContactLoader won't deliver the result for
old contact.

Bug: 30794914
Test: Maually followed repro steps observing new contact is loaded. Also
tested on editing and saving a contact which go through the same logic.

Change-Id: I6c4830ebadaedfdc03616d980999da9da5c46b0b