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/ResourceTable.cpp b/tools/aapt2/ResourceTable.cpp
index fa4b109..deafe20 100644
--- a/tools/aapt2/ResourceTable.cpp
+++ b/tools/aapt2/ResourceTable.cpp
@@ -284,7 +284,7 @@
     }
 
     const auto endIter = entry->values.end();
-    auto iter = std::lower_bound(entry->values.begin(), endIter, config, cmp::lessThan);
+    auto iter = std::lower_bound(entry->values.begin(), endIter, config, cmp::lessThanConfig);
     if (iter == endIter || iter->config != config) {
         // This resource did not exist before, add it.
         entry->values.insert(iter, ResourceConfigValue{ config, std::move(value) });