Android onResume bug

In Android onResume may be called when the lock screen of the phone is
still locked. Thus for example music may play while the screen is
locked.

Info: http://stackoverflow.com/questions/11731285/onresume-being-called-over-and-over-while-phone-screen-is-locked

I changed the code to make notification ```AppDelegate::applicationWillEnterForeground``` just when the application
became visible for the user.