Remove the "+" button on the help menu if no secure vpns can be created

If the provider model is turned on, all new VPNs must have the secure
type. However, there is a system property which is necessary in order to
create secure VPNs. So if the FEATURE_IPSEC_TUNNELS is missing from the
device then we should not enable new 1st party VPNs.

Note, it has been confirmed with the VPN team that we don't expect any
devices released on S to actually be missing this property, but it's
still good to check just in case.

Bug: 176821216
Test: atest -c SettingsUnitTest
Change-Id: I712d342e1970243520612196ba57227b1ff05bbf