Move ContactLoader related code to ContactsCommon

This CL simply moves classes from Contacts into ContactsCommon.

This is needed so that Dialer can use ContactLoader related code
for b/11294679. A ContactLoader will also be needed in the future
to allow InCallUI to download hi-res photos while in call.
Bug: 11294679

Change-Id: Ia033f3ea2406a907a4e71290c9ce810a5583dab8