radv: Fix missing Android platform define.

CC: <mesa-stable@lists.freedesktop.org>
Acked-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
(cherry picked from commit bf33ca75)
Conflicts resolved by Dylan

Conflicts:
	src/amd/vulkan/Android.mk

There is no android.mk support in 18.1 for radv, so only apply the parts
for autotools android builds, which can be used by android
implementations not using android.mk files