[dawn][vulkan] Updates fallback adapter search logic.

- In some configurations, Swiftshader may be loaded through the
  Vulkan loader instead of directly by Dawn. As a result, instead
  of being found through the ICD::Swiftshader path, it is expected
  to be found in the ICD::None path. This change re-orders the
  ICD array so that we always search for Swiftshader first, then
  if we found it, and we are only interested in fallback adapter,
  then we don't bother searching for more. Otherwise, if we have
  yet to find it, we search for it through the ICD::None path as
  well.

Bug: 477694248
Change-Id: I5debc4c90c7b46d0c326b3e40f009183c80a042b
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/287437
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
Commit-Queue: Loko Kung <lokokung@google.com>