aconfig: Add script to generate finalized flag proto.

Takes /prebuilts/sdk/<level>/finalized-flags.txt and creates
aconfig_internal FinalizedFlags proto. For use in automatically adding
the sdk check in exported flag codegen.

Test
1. m convert_finalized_flags_to_proto
2. /path/to/aosp/out/host/linux-x86/bin/convert_finalized_flags_to_proto /path/to/aosp/prebuits/sdk > tmp_flags.pb
3. inspect proto for correctness

Next steps:
-Add genrule which uses this binary
-Read proto output in codegen

Test: manual, see above
Change-Id: I730a1c20347fe1a11fe7213fa25fffbe4ea3fc23
diff --git a/tools/aconfig/aconfig_protos/Android.bp b/tools/aconfig/aconfig_protos/Android.bp
index 62a2b64..080688e 100644
--- a/tools/aconfig/aconfig_protos/Android.bp
+++ b/tools/aconfig/aconfig_protos/Android.bp
@@ -98,3 +98,13 @@
     test_suites: ["general-tests"],
     defaults: ["aconfig_protos.defaults"],
 }
+
+// Internal protos
+
+python_library_host {
+    name: "aconfig_internal_proto_python",
+    srcs: ["protos/aconfig_internal.proto"],
+    proto: {
+        canonical_path_from_root: false,
+    },
+}