Add test for increment operation count on non-default network

Currently, when the caller increment operation count, the count
will be blamed on the active default network even though the
traffic is all generated on other networks. This is kind
of weird. But in order to change the behavior, extend test
coverage first.

Test: atest com.android.server.net.NetworkStatsServiceTest#testOperationCount_nondefault_traffic
Bug: 174123988

Change-Id: Ia5b5aa3601de15bb9ee5a29f6d184d122f1c5352