Fix issue #3272082: Contacts: when going back from edit view,

list UI is not ready yet

This involves some reworking of Loaders.

Loaders, in particular CursorLoader, are now expected to retain
their current data after being stopped.  This allows applications
to keep that data across onStop() -> onStart(), so when the user
returns to the app it doesn't have to wait for the data to reload
and thus cause flicker.

This includes various API changes to better reflect the new
semantics, plus a new LoaderCallbacks method to tell the application
when it is actually time to stop their use of a loader's data.

Note this is somewhat half-done, to help checking in the extensive
application changes that are required without causing build breakage.

Change-Id: Ib4b3bf8185a6da46e7f06ca125521d65e2e380a1