radv: Implement vkGetSwapchainGrallocUsage2ANDROID.
This was implemented in version 6 of the VK_ANDROID_native_buffer extension and we only implement version 5. However, the Android Vulkan loader only checks whether vkGetInstanceProcAddr for the function is not NULL.
This all went wrong when we switched to the layer code from ANV. Because the function may now be different per device, it adds fallback functions that dispatch to the dispatch table. So if we didn't implement the function we still returned a pointer to the dispatch function, which made the Android Vulkan loader believe it was supported.