Unit test automation for Multi-Default Tracking

Unit test automation validating how multiple default networks are
tracked within ConnectivityService when set by network preferences
used with setOemNetworkPreference() API functionality.

Bug: 178632672
Bug: 172347841
Bug: 170068946
Test: atest FrameworksNetTests
Change-Id: Iae1935944214efaa8a21636c55e6d8be816275f7