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: this test passes in master through runtest frameworks-net
Bug: b/78487385
Bug: b/80082746
Change-Id: I884a7a8bd7db3fcd220b785ba9914ac8c77720f0