[Android] Rewrote key event handling

* This fixes duplicate TextEntered events and should properly handle `AKEY_EVENT_ACTION_MULTIPLE` (including key down and key up events; [as far as possible](http://code.google.com/p/android/issues/detail?id=33998)). Unfortunately I don't have/know any IME utilizing this, so can't test it.
* The back button is now mapped to the Escape key (best match without adding new keys).
* The volume buttons are no longer intercepted by SFML and no longer generate events (for now).
* This also fixes event handling eating all events, even unhandled ones.