Define and use default request in Ikev2VpnRunner

Vpn might be included in ConnectivityService mainline module, so
it should prevent calling @hide API. Even if Vpn isn't included
in ConnectivityService mainline module eventually, it's still
fine to use local defined default request instead of calling
@hide API.
Ideally, ConnectivityManager will provide a new API which can
provide the status of physical network even though there is a
virtual network. b/147280869 is used for tracking the new API.

Bug: 172183305
Test: atest FrameworksNetTests
Test: atest CtsNetTestCasesLatestSdk
Test: Manual setup IKEV2 VPN connection.
Change-Id: If76e7497bef4890a74641695c3ebd395795bbe7f