Enforce continuous rendering while pausing Android backends

Backends takes care to setContinuousRendering(true) before calling AndroidGraphics#pause() in their #onPause() method. The problem is that the #onPause() method of the backends are called from the Android UI Thread so a call to AndroidGraphics#onDrawFrame(GL10) may occur (on the GLThread) between thos calls :
  *  graphics.setContinuousRendering(true)
  *  graphics.pause();

AndroidGraphics#onDrawFrame(GL10) may call the ApplicationListener#render() method in the meantime that could set continuous rendering to false as discussed in #2461 

Thus very unlikely, it might appen if the GLThread has the lock on the AndroidGraphics#synch var in the onDrawFrame(GL10) while the UI Thread wait for it in the pause() method... Then the pause() method will set the AndroidGraphics#pause var to true which will never get back to false (it get false in onDrawFrame(GL10) which will never be called as we switch to non continuous rendering...)

hope i'm clear ;-)