use onWindowFocusChanged(bool) instead of onResume()/onPause()

we should use onWindowFocusChanged(bool) to detect if the app is currently
visitible to the user or not. because onPause()/onResume() leads to false
positives on some android versions (e.g. Kindle Fire). if you lock those
devices, while the app running, everything is fine (onPause() is handled
properly). but as soon as you wake up the device, and the lock screen gets
visible, the app is resumed and the music is played (onResume() is
called to early).

this is a) not the best behaviour and b) such games will be rejected on the
samsung app store. onWindowFocusChanged(bool) is, like onPause()/onResume(),
available since API level 1 and it should be safe to simply switch.