vulkan: refactor layer enumeration

Replace Enumerate*Layers by a set of new functions that do not distinguish
instance and device layers.  The new functions are also careful not to
pollute the rest of the loader with std containers.

There should be no user-visible change.

Bug: 27911856
Change-Id: I4790fadc1aa2ea934a4628bce55dd45892f15e0b
3 files changed