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/process/SymbolTable.cpp b/tools/aapt2/process/SymbolTable.cpp
index 7309396..9a8b263 100644
--- a/tools/aapt2/process/SymbolTable.cpp
+++ b/tools/aapt2/process/SymbolTable.cpp
@@ -55,7 +55,7 @@
     if (name.type == ResourceType::kAttr || name.type == ResourceType::kAttrPrivate) {
         const ConfigDescription kDefaultConfig;
         auto iter = std::lower_bound(sr.entry->values.begin(), sr.entry->values.end(),
-                                     kDefaultConfig, cmp::lessThan);
+                                     kDefaultConfig, cmp::lessThanConfig);
 
         if (iter != sr.entry->values.end() && iter->config == kDefaultConfig) {
             // This resource has an Attribute.