Convert the sdk proto genrule to use soong var

Instead of baking it into the Android.bp file, define the canonical
value in build/make and reuse it here.

Bug: 195281582
Test: m current_sdkinfo and inspect
Change-Id: Ifd426ca602c14e7a5c5605622da11c992d0c5c59

tools/Android.bp

@@ -49,7 +49,11 @@ gensrcs {
     name: "cur_sdkinfo_src",
     srcs: [""],
     tools: [ "gen_sdkinfo" ],
-    cmd: "$(location) -v 1 -o $(out)",
+    product_variables: {
+        platform_sdk_extension_version: {
+            cmd: "$(location) -v %d -o $(out)",
+        }
+    }
 }
 
 prebuilt_etc {