[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; see comment). 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.
Merge pull request #646 from MarioLiebisch/android-keyevents

[Android] Rewrote key event handling