Fix bit-rotted NetworkStatsServiceTest.

Breakages:

-ag/574873 - Renders testReportXtOverDev obsolete as this is no longer
             a supported mode. Test has been removed.
-ag/600223 - Tests were sending a CONNECTIVITY_ACTION bcast to trigger
             a call to updateIfaces(), but the listener was removed.
             Tests now call forceUpdateIfaces() directly.
-ag/648284 - Calls to get VPN info were not mocked.

Change-Id: I309f2b5d006549104cb1d3cb83e99363dd6dac16