Restore legacy VPN stats dialog.

Was originally removed in ag/522961, but restoring to keep legacy VPN
behavior the same from within VpnSettings. This dialog is only
accesible from VpnSettings and so should only ever be shown for legacy
VPNs.

Bug: 17164793
Change-Id: I06c4e136e1023b8f84edfd15a15264d2e41d325b