Use thumbnail for color extraction

Previously, Contacts was downscaling the full sized image returned from
ContactsLoader. The resulting thumbnail was different enough from the thumbnails
synced by ContactSyncAdapter that about 20% of contact photos had different
extracted colors in Bugle and Contacts.

Bug: 17258486
Change-Id: I8e2cd239195a18afbe0610e47104d7cbe63d3f23