Add Ikev2VpnProfile as public API

This change adds configuration classes for Platform VPNs, with the
extensibility for further platform VPNs to be added in future.

Profile validity is checked upon construction, and upon changing
conversion from VpnProfile instances. Internal storage and method calls
all use VpnProfiles to allow for reuse of existing infrastructure.
However, when Platform VPN implementations are started, the internal
VpnProfile will be converted back into the respective
PlatformVpnProfile for validity checking.

Bug: 143325939
Test: Compiles, FrameworksNetTests passing.
Change-Id: I3c375fb08c132fc062e893c375f5c36460122162