Launch theme fix

Disabled "window preview" so that the theme is always correct on the app's launch. 

BTW, we weren't really benefited by that mechanism since our onCreate() is pretty lightweight and the heavy lifting is being done asynchronously in the DirectoryScanner class. To be honest, I haven't tested this on a slow device, but my opinion is that this is a change for the good. 

Feel free to close the pull request if you disagree, it was a single line change anyway :)