Rename NetworkUsageStatsTest and move to networking CTS tests.

NetworkUsageStatsTest is the CTS test for the NetworkStatsManager
public APIs. Rename it to NetworkStatsManagerTest, since that is
a better name for it.

Also move it to the networking CTS tests, where it will run on
presubmit on every supported Android version (S, T), and can
access hidden networking APIs. This is important because
NetworkStatsManager is being mainlined in T.

Bug: 204830222
Test: atest CtsNetTestCases:NetworkStatsManagerTest
Change-Id: Ie0fd267aa8bf94594fcc939a8493bef8ab14d3fe