AAPT2: Switch to protobuf for intermediate format

Without needing to conform to the runtime data format,
it is much easier to add new features such as debugging symbols
and carrying over product data to link time.

This also simplifies the runtime format parser and serializer,
which will change much less frequently than the protobuf intermediate
format.

Change-Id: I209787bbf087db0a58a534cb8511c51d21133e00
diff --git a/tools/aapt2/ResourceValues.h b/tools/aapt2/ResourceValues.h
index 8e317db..dc2e28e 100644
--- a/tools/aapt2/ResourceValues.h
+++ b/tools/aapt2/ResourceValues.h
@@ -154,8 +154,8 @@
     bool privateReference = false;
 
     Reference();
-    Reference(const ResourceNameRef& n, Type type = Type::kResource);
-    Reference(const ResourceId& i, Type type = Type::kResource);
+    explicit Reference(const ResourceNameRef& n, Type type = Type::kResource);
+    explicit Reference(const ResourceId& i, Type type = Type::kResource);
 
     bool flatten(android::Res_value* outValue) const override;
     Reference* clone(StringPool* newPool) const override;