[mle] enable announce sender by default (#6741)

The `OPENTHREAD_CONFIG_ANNOUNCE_SENDER_ENABLE` feature has been
incorporated into the Thread 1.2.1 draft specification.

src/core/config/announce_sender.h

@@ -44,7 +44,7 @@
  *
  */
 #ifndef OPENTHREAD_CONFIG_ANNOUNCE_SENDER_ENABLE
-#define OPENTHREAD_CONFIG_ANNOUNCE_SENDER_ENABLE 0
+#define OPENTHREAD_CONFIG_ANNOUNCE_SENDER_ENABLE 1
 #endif
 
 /**

src/core/thread/announce_sender.hpp

@@ -217,7 +217,7 @@ public:
      * @param[in]  aInstance   A reference to the OpenThread instance.
      *
      */
-    AnnounceSender(Instance &aInstance);
+    explicit AnnounceSender(Instance &aInstance);
 
     /**
      * This method notifies the `AnnounceSender` that a MLE Announcement message was received with a current timestamp

tests/toranj/openthread-core-toranj-config.h

@@ -399,14 +399,6 @@
  */
 #define OPENTHREAD_CONFIG_CHANNEL_MANAGER_THRESHOLD_TO_CHANGE_CHANNEL (0xffff * 10 / 100)
 
-/**
- * @def OPENTHREAD_CONFIG_ANNOUNCE_SENDER_ENABLE
- *
- * Define as 1 to enable `AnnounceSender` which will periodically send MLE Announce message on all channels.
- *
- */
-#define OPENTHREAD_CONFIG_ANNOUNCE_SENDER_ENABLE 1
-
 /**
  * @def OPENTHREAD_CONFIG_CHILD_SUPERVISION_ENABLE
  *