For #18426: add DefaultActivityLifecycleCallbacks.

We'll extend this in subsequent commits.

app/src/main/java/org/mozilla/fenix/android/DefaultActivityLifecycleCallbacks.kt

@@ -0,0 +1,25 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+package org.mozilla.fenix.android
+
+import android.app.Activity
+import android.app.Application
+import android.os.Bundle
+
+/**
+ * An inheritance of [Application.ActivityLifecycleCallbacks] where each method has a default
+ * implementation that does nothing. This allows classes that extend this interface to have
+ * more concise definitions if they don't implement some methods; this is in the spirit of
+ * other `Default*` classes, such as [androidx.lifecycle.DefaultLifecycleObserver].
+ */
+interface DefaultActivityLifecycleCallbacks : Application.ActivityLifecycleCallbacks {
+    override fun onActivityCreated(activity: Activity, bundle: Bundle?) {}
+    override fun onActivityStarted(activity: Activity) {}
+    override fun onActivityResumed(activity: Activity) {}
+    override fun onActivityPaused(activity: Activity) {}
+    override fun onActivityStopped(activity: Activity) {}
+    override fun onActivitySaveInstanceState(activity: Activity, bundle: Bundle) {}
+    override fun onActivityDestroyed(activity: Activity) {}
+}