Re-Add VPN-"Settings" button to QSFooter dialog

Button was removed during a redesign of the dialog in the case
of active device owner. However, when no DO is set, the button should
still be there.

Also, when VPN configuration is restricted, do not show the button, and
also do not show the link that is visible in the DO-version of the dialog.

Bug:34156553
Bug:29748723
Test: manual

Change-Id: I177afdc94dc396754fc7703e4316edaa7ce104cd
(cherry picked from commit a99a3f70)