Properly implement stubhal's vkGetInstanceProcAddr()

It used to always return a crashing-NoOp function for unknown functions.  Now,
it attempt to follow the spec and return either NULL or a non-crashing-NoOp
function where appropriate.

Test: VulkanFeaturesTest
Bug: 68779289
Change-Id: I98f9fd2a0a1bef4a985c46dbce93216488da82e7
(cherry picked from commit 53333057f31bf91765b930b12ec44e9af1bd18f2)