android: workaround the inability to log JS messages in crosswalk

Instead, provide an appropriate function as a JS interface.
android: add documentation on how to compile with crosswalk
android: update webview JS interface as per XWALK-1596

We should now use `org.xwalk.core.JavascriptInterface` to declare JS
interfaces.
android: display wait screen as a dialog

Otherwise, the webview is not active and nothing happens. This is
because the webview is implemented as a surface and the surface is not
activated unless it is visible...
android: use a transparent actionbar
android: don't allow user to cancel wait screen
android: make the wait screen full screen
android: access preferences using the back key

This is simpler than to use the action bar inside a dialog.
Revert "android: use a transparent actionbar"

This reverts commit fa85d80c6624c736b64398a506b2f1226ae39500.

The transparent actionbar is also transparent when in the preferences
dialog. This is not convenient.