Consolidate all work in the WorkHandler.

Fix flakey tests, and remove indeterminism in tests but asserting all
latches successfully countdown.

Bug: 68030053
Test: runtest --path
frameworks/base/packages/SettingsLib/tests/integ/src/com/android/settingslib/wifi/WifiTrackerTest.java

Change-Id: I68f108929d563bc323a28bdfb3c9eb514630630b