Make Cron.CronPattern.FIELDS immutable

The List returned from Arrays.asList() is able to mutate the underlying
array.

src/com/google/enterprise/adaptor/CronScheduler.java

diff --git a/src/com/google/enterprise/adaptor/CronScheduler.java b/src/com/google/enterprise/adaptor/CronScheduler.java
index 215fb89..f0f260a 100644
--- a/src/com/google/enterprise/adaptor/CronScheduler.java
+++ b/src/com/google/enterprise/adaptor/CronScheduler.java
@@ -163,7 +163,8 @@
   }
 
   private static class CronPattern {
-    private static final List<Field> FIELDS = Arrays.asList(Field.values());
+    private static final List<Field> FIELDS
+        = Collections.unmodifiableList(Arrays.asList(Field.values()));
 
     private static enum Field {
       MINUTE(0, 60, Calendar.MINUTE, 0),