Introduce EXITED state, along with RUNNING/PAUSED.

Once we're EXITED, we have to stop dispatching invokeLater() runnables as that
could result in attempting to do GL things after our activity has exited and a
new instance of our activity has started and is doing GL things. GL gets very
unhappy when we do that.

Thanks to @ruslansennov for the patch which I munged into this one.

I also nixed AndroidPlatform.paused() as it was not used.