[TIMOB-2626] Android: density images don't get copied to res folder if any JS file has changed and app is rebuilt

(Bug is on Master, not in the released 1.5.1.) There is something wrong with app build as it pertains to density images getting copied out to res/drawable-mdpi, etc. To reproduce:

• Create a new Titanium project.
• Create the folder Resources/android/images/medium in that project.
• Copy Resources/KS_nav_ui.png to Resources/android/images/medium/test.png
• Replace app.js with this:

Titanium.UI.setBackgroundColor('#000');
var win = Titanium.UI.createWindow({  
    title:'Test',
    backgroundColor:'#fff',
    fullscreen: false,
    exitOnClose: true
});

win.add(Ti.UI.createImageView({image: 'images/test.png', height: 50, width: 50}));
win.open();

• Launch the app. (Ignore the build warnings about density-specific images.) You should see the image fine when the app loads in the emulator/phone.
• Edit app.js and just put anything in it,like a new line at the bottom.
• Launch the app again. You won't see the image.