vulkan: do not use exported functions internally

Our vkGet*ProcAddr uses the exported functions.  They will break when any
of the exported functions are overridden (e.g., through LD_PRELOAD).

Unexport and move all exported functions to vulkan::api namespace.
Re-export them by having vkFoo as a wrapper to vulkan::api::Foo.

Another option is to re-export vulkan::api::Foo by having vkFoo as an
alias using __attribute__((alias)).  That results in smaller binaries.
But we will not be able to catch mismatches between vulkan.h and
vulkan.api.

To avoid future breakage, define VK_NO_PROTOTYPES for all files except
api_gen.cpp.

Bug: 28886971
Change-Id: I08fde7ebb247f8c7e040ccf812b40b02094d3c7f