Use MonitorThreadLeak in ConnectivityService tests

Fix current leaks in ConnectivityServiceIntegrationTest, which leaked
the ConnectivityService and CarrierPrivilegeAuthenticator handler
threads, and enable the checks.

Using @DevSdkIgnoreRunner.MonitorThreadLeak ensures that tests properly
cleanup threads (in particular handler threads), as many leaked threads
could affect other tests.

Bug: 310659032
Test: atest --rerun-until-failure 20
Change-Id: Ia62d33d86217fef1d03532bb51c890a90430a70e