Disable Checkstyle checks on auto-generated files.

Add a check to ignore changes to automatically generated classes.

Bug: None
Test: Tried to run checkstyle locally on a file that containted the comment
      and checkstyle script showed no warnings even though that file had issues.
Change-Id: If9085948f1c224dbb53255b2ba1601511ef769dd

tools/checkstyle/android-style.xml

@@ -9,6 +9,10 @@
     <property name="onCommentFormat" value="CHECKSTYLE:ON IndentationCheck"/>
     <property name="checkFormat" value="IndentationCheck"/>
   </module>
+  <module name="SuppressionCommentFilter">
+    <property name="offCommentFormat" value="CHECKSTYLE:OFF Generated code"/>
+    <property name="onCommentFormat" value="CHECKSTYLE:ON Generated code"/>
+  </module>
   <module name="TreeWalker">
     <module name="AvoidStarImport">
       <property name="severity" value="error"/>