Add unit test on getting initial BSSID in IpClientTest.

Refactor setInitialBssid method to static and be able to call it
in the test directly. Unit test is much easier and faster to run
than integration test, less flaky. Also delete the integration
tests on this topic.

Bug: 185202634
Test: atest NetworkStackTest NetworkStackIntegrationTests
Change-Id: I16eedbf9b9705980a96984323aaab1af14d3d65d