Add @FunctionalInterface annotation

Author: lfkellogg

firebase-appdistribution/src/main/java/com/google/firebase/appdistribution/impl/TaskCache.java

@@ -23,6 +23,7 @@
 class TaskCache<T extends Task> {
 
   /** A functional interface for a producer of a new Task. */
+  @FunctionalInterface
   interface TaskProducer<T extends Task> {
 
     /** Produce a new Task. */