Bug 826839 - Fix Add Contact Btn visibility after a call and simplify it...

Restructured the code for updating the status (i.e. 'disabled') of the Add Contact button from the dialer so that it is linked with the Delete (digit) button's visibility update procedure.  

The remaining redundant method (updateAddContactStatus) was removed since it is not used by any other method and its only purpose was to update the status of the Add Contact button.