Added thin object layer around contact data

This refactoring abstracts out the need to directly
refer to Contacts database columns throughout the code.  Instead,
all of this information is retained in getter/setter methods
within the Contact, RawContact, and DataItem classes and
sub-classes.

ContactLoader.Result class has been pulled to the top level as
the Contact class.

The Entity class has been removed and replaced with a RawContact
class, with getters/setters to raw contact information.
Renamed EntityDelta to RawContactDelta for better understandability
as well as adding getters/setters for specific fields in the
ValuesDelta nested class within EntityDelta.  EntityDeltaList
and EntityModifier have been renamed to RawContactDeltaList and
RawContactModifier with the methods using the RawContact class
directly rather than the Entity class.

Data items for a raw contact are represented by a DataItem object
with specialized getters/setters for subclasses of DataItem.
(e.g., EmailDataItem, PhoneDataItem. etc.).  DataItem is a wrapper
around ContentValues.  This abstracts away the ContactsContract
column fields into getters/setters.

The above refactoring is accompanied with changes throughout the
codebase to use the new Contact, RawContact, and DataItem classes.

Change-Id: I31c1dccd724e9652f9d0af78ca81feb6c5acd71d