Create "icd" subdir in angledata directory where the Vulkan mock ICD
json file is stored. This prevents potential loader conflicts with the
layer json files. This change means that when running with the mock
ICD a user will need to set their VK_ICD_FILENAMES env variable to now
point to "<build_out_dir>/angledata/icd/VkICD_mock_icd.json" and make
sure that the built mock icd library resides in the system's shared
object search path.

Bug: angleproject:2159
Change-Id: I802bb61a06a2f370bb1fa7a28d9c3548e5f94ecf
Reviewed-on: https://chromium-review.googlesource.com/914819
Commit-Queue: Tobin Ehlis <tobine@google.com>
Reviewed-by: Yuly Novikov <ynovikov@chromium.org>