Temporarily disable 2 unratified Khronos extensions.

Test: Ran the following CTS test cases, which now say "NotSupported":
dEQP-VK.wsi.android.surface.query_capabilities2
dEQP-VK.wsi.android.surface.query_formats2
dEQP-VK.wsi.android.shared_presentable_image.scale_none.demand
dEQP-VK.wsi.android.shared_presentable_image.scale_none.continuous
dEQP-VK.wsi.android.shared_presentable_image.scale_up.demand
dEQP-VK.wsi.android.shared_presentable_image.scale_up.continuous
dEQP-VK.wsi.android.shared_presentable_image.scale_down.demand
dEQP-VK.wsi.android.shared_presentable_image.scale_down.continuous

Disable the Khronos/Vulkan VK_KHR_get_surface_capabilities2 and
VK_KHR_shared_presentable_image extensions until the May 5th
ratification of these two extensions.  A macro/ifdef's are used to allow
easy enablement of the extensions for internal testing and bug fixes.

V2: convert preprocessor flag to constant [chrisforbes]

Change-Id: Ic41360e42561a822d51e1d7fc3172c76c12e5bde