Use MonitorThreadLeak in CSTest related tests

Fix current leaks in CSTest related tests, which leaked the
ConnectivityService and AlarmManager 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.

Fix: 308520131
Test: atest ConnectivityCoverageTests
Change-Id: Ia6fb2c97f7a1740bafc0ab549440fdbeb1d5fad6