Vulkan: Do not build mock ICD on Android. This is causing a GN build error on our Android bots. Bug: angleproject:2471 Bug: angleproject:2482 Change-Id: I35ffc7972effb9c9ea26ab96cf23985eba1702c3 Reviewed-on: https://chromium-review.googlesource.com/1016700 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Jamie Madill <jmadill@chromium.org>