ConnectivityServiceTest: mark flaky test as such

This patch extracts into its own independent test a test sub-block looking
for a race condition when not waiting on handlers to become idle:
there is no way to prevent the race from not happening when looking for
it this way. This makes the test flakky.

This new independent test is tagged with @FlakkyTest(tolerance = 3).

Test: ConnectivityServiceTest passes, with higher probability.
Bug: 31479480
Change-Id: Ic0e2ffbe669f2ea8cc62e8a0d4763703368db019