VPN: Explicitly handle IAE from updating underlying network

IpSecTunnelInterface#setUnderlyingNetwork will throw IAE when the
underlying network is not functional and has null LinkProperties.
This commit updates IKEv2 VPN to explicitly handle this exception.

Bug: 240112879
Test: VpnTest, VpnManagerTest, VpnManagerServiceTest, Ikev2VpnTest
Change-Id: I962ee07a5b0e38a07653c25349e41ae47623e7f6