Add VpnManger API surface

This change adds the VpnManager, which will be used by apps to install
profiles for all platform VPN types (currently only IKEv2).

Bug: 143325939
Test: Compiles, FrameworksNetTests passing.
Change-Id: I57f854d0a5b18358f3541c24ca0cd8aed03fd7a1
5 files changed