Replace our own MyWorkerThread class by using HandlerThread class

Android's HandlerThread already has a Looper, so can avoid hooking up one.
(Leaving MyWorkerThread class as example of adding a Looper to a Thread.)

Improve comments for the two Java methods that are bound to WebView
JavaScript and so run in its separate thread for WebView's JavaScript/Java
binding.

Add some more code to try to prevent leaks.
Update README after upgrading Chrome browser on dev Mac host to v39

That version now matches Chrome for Android browser version on Android device.

Update README to reflect browser version number, and also document edge case
when Chrome DevTool set up for port forwarding could crash app if Chrome for
Android is not already running on Android device.