Fixing formatting of Agora (mobile) dialogs

Changing top and bottom of .oo-ui-window-frame from 1em to 0.

In Agora/Minerva, dialogs should be flush with the top and bottom of
the screen.

See screenshot in bug 69710.

Bug: 69710
Change-Id: I43295af60d16d0beb0440c16945515a29f3cd114