Mock IpConnectivityLog in IpClientTest

IpConnectivityLog which is used to send metrics need NETWORK_STACK
permission. Mock IpConnectivityLog to avoid sending metrics from
tests. Also remove platform cert from the network stack unit test
because it is not necessary for any signature permission anymore.

Bug: 156866746
Test: NetworkStackNextTests, NetworkStackCoverageTests, TetheringTests,
TetheringCoverageTests

Change-Id: Ibff052b30e000de3c5a08d7265c7642d67d9b0d0
Merged-In: Ibff052b30e000de3c5a08d7265c7642d67d9b0d0