android: Clean up meson build, document

Now that a bunch of the Android bits for meson are in place, document how to set up r/w /vendor partitions to be able to replace your drivers on the fly, and add some of the necessary documentation for building out-of-tree drivers. Building DRM drivers using the NDK is still not something I've completed yet.

Some folks that are probably interested: @krh @robclark @alyssa