radv: fix build on Android 7 (v2)

Fixes the following building error:

external/mesa/src/amd/vulkan/radv_android.c:28:10: fatal error:
'vndk/hardware_buffer.h' file not found
         ^~~~~~~~~~~~~~~~~~~~~~~~

(v2) use the existing preprocessor condition #if ANDROID_API_LEVEL >= 26

Fixes: f36b5274 "radv/android: Add android hardware buffer queries."
Reported-and-tested-by: youling 257 <youling257@gmail.com>
Signed-off-by: Mauro Rossi <issor.oruam@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <!6051>
(cherry picked from commit 80c135e6)
34 jobs for staging/20.1 in 20 minutes and 7 seconds (queued for 3 seconds)