Instead of holding an ApplicationContext, JWebCoreJavaBridge

will have a reference of the current window's main
WebView. It is only non-null if the WebView's window
has the focus.

Extract setActive() from onWindowFocusChanged() so
that onAttachedToWindow() can call it directly. The
old way has a mis-matching call to onWindowFocusChanged.

Fix http://b/issue?id=2559152