ANDROID: Rewrite control schemes

This PR is a revised version of #841.
The TOUCH_BACKEND define has been superceded in favor of ENABLE_TOUCH,
which should be enabled on a per-backend basis in the configure script.

With this patchset, every engine can supply its own input handler.
I have extracted the baked-in GRIM/EMI controls into such a handler.

I've also removed a lot of cruft from the Android backend, hopefully
much of it can be reused for the iOS backend.

Comments welcome!