[TIMOB-19511] Add support for platform specific DefaultIcon.png files

Titanium SDK 5.0.0 added support for DefaultIcon.png in the project root directory. This image is intended to be applied to all platforms. Since iOS does not support app icons with an alpha channel, we must allow platform specific images. DefaultIcon.png will continue to live in the project root, but this ticket is meant to add a platform specific override from the <project-dir>/platform/<platform>/DefaultIcon.png file. This logic should be added to to titanium_mobile/node_modules/titanium-sdk/lib/builder.js in the generateAppIcons() function (https://github.com/appcelerator/titanium_mobile/blob/master/node_modules/titanium-sdk/lib/builder.js#L412). It should check for the platform specific DefaultIcon.png, then if that doesn't exist, fall back to the one in the project root before failing out.