Add bug number to a TODO

Add bug number to a TODO since it is dependent on internal changes.
Once those changes hit aosp/master it can be acted on.

Bug: 64015928
Bug: 31008728
Test: make
Change-Id: I097522d07602a715d4519806237f1644fdc5d019
(cherry picked from commit f3a394eeb3408415314c4e816ef4638904940a85)
diff --git a/testing/xts/src/com/android/timezone/xts/TimeZoneUpdateHostTest.java b/testing/xts/src/com/android/timezone/xts/TimeZoneUpdateHostTest.java
index 694a074..a875634 100644
--- a/testing/xts/src/com/android/timezone/xts/TimeZoneUpdateHostTest.java
+++ b/testing/xts/src/com/android/timezone/xts/TimeZoneUpdateHostTest.java
@@ -49,6 +49,7 @@
  *
  */
 // TODO(nfuller): Switch this to JUnit4 when HostTest supports @Option with JUnit4.
+// http://b/64015928
 public class TimeZoneUpdateHostTest extends DeviceTestCase implements IBuildReceiver {
 
     // These must match equivalent values in RulesManagerService dumpsys code.