Rename vars of Ikev2VpnRunner

There is already a var which is named mNetworkCapabilities in Vpn
class. In order to simply distinguish Vpn#mNetworkCapabilities and
Ikev2VpnRunner#mNetworkCapabilities, rename the one in
Ikev2VpnRunner to mUnderlyingNetworkCapabilities, also change
Ikev2VpnRunner#mLinkProperties to mUnderlyingLinkProperties.

Bug: 191413541
Test: atest FrameworksNetTest#VpnTest
Change-Id: I49a5aee159baa1d1583abb94d73da9fbc03ffe07