android: Enable building binder as module

android: Enable building binder as module

Bug-Debian: https://bugs.debian.org/901492

We want to enable use of the Android binder driver to support
Waydroid, but it should not be built-in as that would waste resources
and increase security attack surface on systems that don't need it.

- Change the Makefiles to build the driver as an object with the
  "_linux" suffix (which is what Waydroid expects)
- Change config symbol types to tristate
- Add a module description, because modpost warns about missing
  descriptions

Gbp-Pq: Topic debian
Gbp-Pq: Name android-enable-building-binder-as-module.patch