llvmpipe: Add android platform integration

Add support for for both llvmpipe and lavapipe to run on Android. This MR adds the necessary missing bits to the EGL android platform integration to create a pure SW rasterizating setup without requiring a DRM compatible display device. Instead rendering directly to the buffer gralloc provides to the EGL implementation. It also adds the missing lavapipe platform integration including the VK_ANDROID_native_buffer which is required for Android WSI in Vulkan.

This MR also updates the Android documentation to include instructions for building llvmpipe and lavapipe on Android using the NDK and then include those prebuild libraries in a Android OS image.

I've marked this MR as a draft at the moment, as I would like some feedback on some of the drisw common code changes that were made. I had to modify the image creation to allow the dmabuf passed to the driver to be directly rendered to.

Additionally I would like some feedback on the documentation updates, to make sure they are clear to anyone following them.