Add deprecation notice to SingleLiveEvent.

app/src/main/java/org/thoughtcrime/securesms/util/SingleLiveEvent.java

@@ -36,6 +36,8 @@ import java.util.concurrent.atomic.AtomicBoolean;
  * explicit call to setValue() or call().
  * <p>
  * Note that only one observer is going to be notified of changes.
+ *
+ * @deprecated Use a PublishSubject or PublishProcessor instead.
  */
 public class SingleLiveEvent<T> extends MutableLiveData<T> {