AAPT2: Fix inclusion of comments in R.java javadoc

Comments weren't being copied when merged from the various
resource tables.

Also refactored the JavaClassGenerator to omit a class
if no entries exist for it.

Change-Id: I6eaa89b7b3715bc05403635a2baf0d1db3efd142
diff --git a/tools/aapt2/ResourceValues.h b/tools/aapt2/ResourceValues.h
index 2629153..7ae346a 100644
--- a/tools/aapt2/ResourceValues.h
+++ b/tools/aapt2/ResourceValues.h
@@ -91,7 +91,7 @@
      */
     virtual void print(std::ostream* out) const = 0;
 
-private:
+protected:
     Source mSource;
     std::u16string mComment;
 };