Fix FindBugs error in DelayedListenerRegistration#getInstance

Fix FindBugs error in DelayedListenerRegistration#getInstance

The ObjectRegistration interface was recently changed to annotate
getInstance with @Nonnull to promise it will not return a null.
However DelayedListenerRegistration could return null if the delegate
is not set yet. In reality, we do not and should not ever call this
method on DelayedListenerRegistration instances so I changed it to
throw UnsupportedOperationException to make it explicit and to avoid
the FindBugs error.

Change-Id: I9fe374b23336d8ade65b2f1b697d93f50a090df9
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>