Drop non-API headers from the NDK vulkan headers.

The other headers in this directory are just for the platform and
shouldn't be in the NDK. Fortunately we never exposed the symbols in
the stub libraries so users couldn't actually do anything with the
other headers, but we shouldn't be exposing these headers anyway.

Test: make checkbuild
Bug: http://b/36224616
Change-Id: I9e162964e349a256e7c7ad25eca3c33c5631e2c8