Fix testRegisterUsageCallback failure in setUp

LocalServices.addService in NetworkStatsService is currently failing
with IllegalStateException "Overriding service registration". Setting up
LocalServices in the test to avoid this issue might be possible, but
moving the registration to the only non-test caller of that constructor
as done here solves the issue and avoids side-effects from a constructor.

Test: atest FrameworksNetTests does not choke on this test
Bug: b/78487385
Bug: b/80082746
Change-Id: I5dba98fc79aec0800c8b71e6c7e23d1cfbcae852
Merged-In: I884a7a8bd7db3fcd220b785ba9914ac8c77720f0
(Clean cherry-pick of go/ag/4061255)

tests/net/java/com/android/server/net/NetworkStatsServiceTest.java

@@ -227,9 +227,6 @@ public class NetworkStatsServiceTest {
 
     @After
     public void tearDown() throws Exception {
-        // Registered by NetworkStatsService's constructor.
-        LocalServices.removeServiceForTest(NetworkStatsManagerInternal.class);
-
         IoUtils.deleteContents(mStatsDir);
 
         mServiceContext = null;