Fix race condition / test flakiness.

The potential for a race condition exists in WifiTrackerTest,
which intermittently manifests in APCT. By waiting for all handlers
to be idle before asserting our test verification, we can fix this
flaky test. Also added more verification steps to test code to more
precisely indicate where a test may be failing.

Bug: b/64047370
Test: runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/WifiTrackerTest.java

Change-Id: I597fd7dc29e7f3b0d13c8cdebfa208326a64f507