[ggj][engx] fix: use singleton in HtmlEscaper (#272)

* fix: use singleton in HtmlEscaper

* fix: linter

* fix: merge

src/main/java/com/google/api/generator/engine/escaper/HtmlEscaper.java

@@ -18,10 +18,12 @@
 import com.google.common.escape.Escapers;
 
 public class HtmlEscaper extends Escaper {
+  private static final HtmlEscaper SINGLETON = new HtmlEscaper();
+
   // Based on the observation of the generated java files, we escape the following
   // five characters by html escaper. We do not directly use guava HtmlEscapers here because
   // it only escapes`<>&\"'` as specified by HTML 4.01.
-  private static final Escaper escaper =
+  private static final Escaper charEscaper =
       Escapers.builder()
           .addEscape('<', "&lt;")
           .addEscape('>', "&gt;")
@@ -34,10 +36,10 @@ private HtmlEscaper() {}
 
   @Override
   public String escape(String sourceString) {
-    return escaper.escape(sourceString);
+    return charEscaper.escape(sourceString);
   }
 
   public static String escaper(String source) {
-    return new HtmlEscaper().escape(source);
+    return SINGLETON.escape(source);
   }
 }