android: anv: fix generated files depedencies (v2)

Fix anv_extrypoints.{c,h} and anv_extensions.{c,h} missing dependencies
Rename the variable labels according to targets and python scripts
Align the building rules as per Automake for simplification

Fixes building errors during rebuils due to missing dependencies

(v2) Fixed a missing $(VULKAN_API_XML) reference

Fixes: 9a508b71 ("android: anv/extensions: fix generated sources build")
Fixes: dd088d4b ("anv/extensions: Generate a header file with extension tables")
Signed-off-by: Mauro Rossi <issor.oruam@gmail.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
Cc: "19.0" <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 14e7e26a)