Re-land fixes angle_end2end_tests disabling Vulkan.

This moves the build configuration into the Vulkan back-end dir.
This should be a little easier to maintain as all Vulkan-related
config is in one place.

Note that this should not interfere with Skia's build as they do
not import the Vulkan back-end sources.

One additional possiblity that this enables is testing other
compile-time permutations of the Vulkan back-end more easily. For
example we could make a simple change to enable compile testing
of the Vulkan back-end with custom command buffers disabled.

Also fixes a few errors affecting less tested configs.

Bug: angleproject:3943
Change-Id: I0161668abcc58fcf529dde120998d4b99445fdd5
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1838454Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>