This fixes a bug in SystemInfo_vulkan where we were using the
System search path for the Vulkan loader when we prefer using the
custom ANGLE loader.

Re-land fixes a bug where we would try to load the custom
libVulkan on Android and other platforms where we should be using
the system version.

Bug: chromium:1219969
Change-Id: I34b592fb87cbddfd02c837a17942cac54c85d9d1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3007265Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>