Change WifiListenerWrapper to WifiListenExecutor.

The executor no longer instantiates it's on main thread handler, and
instead uses ThreadUtils.postOnMainThread.

Also clean up WifiTracker tests, since the custom Handlers have been
removed, and the broadcast receiver method is invoked directly in the
tests, it is no longer necessary to wait for handlers to finish processing
messages.

Although the only asynchronous source code (invocation of WifiListener
methods) is now mockable via WifiListenerExecutor injection,
CountDownLatches used in cases where verifying multiple mock invocations
would be confusing due to test setup behavior.

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

Change-Id: I485c62d81ec2cdd3d53ed3e258ae6b40837cdc51