Ensure that the copy notification is always cancelable.

If the CopyService dies or is stopped for whatever reason, the cached
"current" job ID will go to null.  In that case, the cancel intent will
be ignored, and the copy notification can never be dismissed.  Adding a
null check to the handleCancel method of CopyService fixes this.

BUG=21365043
Change-Id: I0615328f6033a313fec79f8b63291c7f7373ea0f