Disable angle_shared_libvulkan on Mac since we are the only client.
Re-add codepaths to support this.

Bug: angleproject:4477
Change-Id: Ie128c83adaae741636541bbfd6105d160d874a8d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2102954
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Tobin Ehlis <tobine@google.com>