email: display a "load more" button

If the message is partial loaded, then show a "load more" button instead of an attachment
bar. A partial message doesn't implies that the message contains attachments or that
all attachment were loaded.

Change-Id: Icac484a04d086c0cf3b3099b071c0c123aec2344
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>
email: prevent NPE in EmailProvider

Check that we have loaded a valid cursor. After deleting an account some query could
return a null cursor.

Change-Id: Ief2f948a61a308d9e17af80730612a02306336e5
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>