ConnectivityServiceTest: remove flaky waitForIdle test.

This patch removes testNotWaitingForIdleCausesRaceConditions() from
ConnectivityServiceTest because it is in nature flaky and prevents using
ConnectivityServiceTest as a patch presubmit check. Estimated failure
rate is 1/15 on Nexus 6p.

This patch also simplifies how ConnectivityServiceTest waits for
handlers to become idle by removing IdleableHandlerThread and turning it
into a signle static method.

Test: $ runtest frameworks-net
Bug: 31479480
Change-Id: I2d78709cbb61d5d01cd59cff326469417f73f1ab