Add always-on VPN support for platform VPNs

This commit allows Platform VPNs to be started as part of always-on
mode.

Test: FrameworksNetTests passing, new tests added in subsequent CL
Test: Manually tested.
Change-Id: I5eda88e5b406a0e425eb7424665cf702e0979324
Merged-In: I5eda88e5b406a0e425eb7424665cf702e0979324