Email: Hide the feedback button if nowhere to feedback

The string value of email_feedback_uri in strings.xml presents where
to submit the feedback. If it is empty or invalid, shall hide the
feed back button.

Add the switch to check email_feedback_uri to show feedback button for
the Email module. Add the switch to set the capability for UnifiedEmail
to use.

CR-fixes: 667335

Change-Id: Ia01350417b0b020b03093212ea7ef0ff2d655fe9