[Android] Add a configuration option to disable the back button

In our application, the logic is very complicated as far as when and if you can go back, so we need a way to kill the back button dead that works across a ton of devices. This patch provides that.

Note that this does not actually document the configuration option or put it in the default rhoconfig.txt. Whoever does that should probably do that, and also document CAPath and CAFile while they're at it. :)

- Keith Gable @ DecisionPoint (kgable@decisionpt.com)