Combine extinst-name and extinst-output-base into one arg. (#3200)

* Combine the extinst-name and extinst-output-base into one arg.

Some build systems such as Android blueprints require that the inputs
and outputs of generator scripts are all provided as arguments.  These
two arguments to generate_language_headers.py are combined to form the
output path in the script.  This change simply lets the user provide the
whole output path as an argument.

* Fix typo in build_defs.bzl and update Android.mk
diff --git a/BUILD.gn b/BUILD.gn
index b7cde34..1337059 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -186,21 +186,19 @@
     script = "utils/generate_language_headers.py"
 
     name = invoker.name
-    extinst_output_base = "${target_gen_dir}/${name}"
+    extinst_output_path = "${target_gen_dir}/${name}.h"
 
     args = [
-      "--extinst-name",
-      "${name}",
       "--extinst-grammar",
       rebase_path(invoker.grammar_file, root_build_dir),
-      "--extinst-output-base",
-      rebase_path(extinst_output_base, root_build_dir),
+      "--extinst-output-path",
+      rebase_path(extinst_output_path, root_build_dir),
     ]
     inputs = [
       invoker.grammar_file,
     ]
     outputs = [
-      "${extinst_output_base}.h",
+      "${extinst_output_path}",
     ]
   }
 }