Add additional strings to differentiate between single and multiple

insecure vpns

Although in english, the strings can be "N not secure", in other
languages translators have requested that we add "VPN" or "VPNs" after
the number for extra clarity. Thus, we split the string into two
depending on whether there is just one or more than one insecure VPN.

Bug: 184167494
Test: atest -c SettingsUnitTests
Change-Id: I93f0cbec5f22ffcac70ddc2ecf640729eef29c04