Add NET_CAPABILITY_NOT_VPN when requesting test network

Add NET_CAPABILITY_NOT_VPN can prevent Ikev2VpnNetworkCallback
receiving onAvailable when VPN is connected. Otherwise, the VPN
network will be misused as underlying network to establish VPN
connection again, which is a wrong behavior.

Bug: 228574221
Test: atest CtsNetTestCases:Ikev2VpnTest
Change-Id: Iac85caa0f5bd37a7e79c14fe796737d7b36fa4bd