Bug 2045793 - Add feature update callback registration to FeatureHolder (#7406)

These methods allow methods for adding and removing update callbacks
with Nimbus without passing string constants around in client code,
making it less likely to cause misregistration due to typos.

Author: freshstrangemusic

components/nimbus/android/src/main/java/org/mozilla/experiments/nimbus/internal/FeatureHolder.kt

@@ -134,6 +134,23 @@ class FeatureHolder<T : FMLFeatureInterface>(
         }
     }
 
+    /**
+     * Add a callback to be called when the feature updates.
+     *
+     * This **must** be called after Nimbus is initialized, otherwise the
+     * callback will not be registered.
+     */
+    fun addUpdateCallback(callback: () -> Unit) {
+        getSdk()?.getFeatureUpdateDispatcher()?.addCallback(featureId, callback)
+    }
+
+    /**
+     * Remove a feature update callback.
+     */
+    fun removeUpdateCallback(callback: () -> Unit) {
+        getSdk()?.getFeatureUpdateDispatcher()?.removeCallback(featureId, callback)
+    }
+
     /**
      * Is this feature the focus of an automated test.
      *