[TIMOB-1975] Android: Showing JS error dialog (rhino dialog) can FC app if activity has finished

Attempting to show an error dialog inside the JS (our doRhinoDialog in TiContext) FCs the app if the activity belonging to the context that is attempting to show the dialog has finished.

This can happen, for example, if a user backs out of a heavyweight window while a separate thread (like an XHR callback, which is how we found out about this problem) is running JS in that window's context and comes across an error.

For a failcase, use this app.js and then win.js.

app.js:

Titanium.UI.setBackgroundColor('#000');
Titanium.UI.createWindow({  
    title:'Test',
    backgroundColor:'#fff',
    fullscreen: true,
    url: 'win.js'
}).open();

win.js:

var win = Ti.UI.currentWindow;
win.addEventListener('close', function(){
    Ti.API.info(win.nonsense.nonsense);
});

The FC is this exception:

android.view.WindowManager$BadTokenException: Unable to add window -- token android.os.BinderProxy@45d433c8 is not valid; is your activity running?
at android.view.ViewRoot.setView(ViewRoot.java:509)
at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:177)
at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:91)
at android.view.Window$LocalWindowManager.addView(Window.java:424)
at android.app.Dialog.show(Dialog.java:241)
at org.appcelerator.titanium.TiContext$1.run(TiContext.java:811)
at android.os.Handler.handleCallback(Handler.java:587)
at android.os.Handler.dispatchMessage(Handler.java: