Refactor proto in preparation for java proto support

Test: m -j checkbuild
Change-Id: Idf00ea0bacb2777458f9af2c7eb47e1e1854eeba
diff --git a/Android.bp b/Android.bp
index 89bd6ea..0875a2b 100644
--- a/Android.bp
+++ b/Android.bp
@@ -53,6 +53,7 @@
         "android/package_ctx.go",
         "android/paths.go",
         "android/prebuilt.go",
+        "android/proto.go",
         "android/register.go",
         "android/testing.go",
         "android/util.go",