anv: do not advertise AHW support if extension not enabled

Fixes following failing vk-gl-cts cases on Linux desktop:

dEQP-VK.api.external.memory.android_hardware_buffer.suballocated.buffer.info dEQP-VK.api.external.memory.android_hardware_buffer.suballocated.image.info dEQP-VK.api.external.memory.android_hardware_buffer.dedicated.image.info dEQP-VK.api.external.memory.android_hardware_buffer.dedicated.buffer.info

Fixes: 517103ab "anv/android: add ahardwarebuffer external memory properties" Reported-by: Juan A. Suarez jasuarez@igalia.com Signed-off-by: Tapani Pälli tapani.palli@intel.com