DO NOT MERGE: 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

(cherry picked from commit c8c10277)

Change-Id: I32632bb763585c23f1de5bb1811a384a35bf9f64