fix: give TimestampFormat enum public accessor

Author: raphkim

aws-android-sdk-core/src/main/java/com/amazonaws/transform/TimestampFormat.java

@@ -20,7 +20,7 @@
 /**
  * Simple enum to differentiate timestamp format standards.
  */
-enum TimestampFormat {
+public enum TimestampFormat {
     /**
      * In epoch seconds.
      */