anv: use the correct format on Android

Per https://android.googlesource.com/platform/frameworks/native/+/master/vulkan/libvulkan/swapchain.cpp#745


the format Android requires is R5G6B5, and we have it backwards here.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Part-of: <mesa/mesa!4898>