unifiedemail: 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.

Require http://review.cyanogenmod.org/#/c/67523/

Change-Id: I8998ac7f7c46d7e6698ee471fb89185504d1ecf0