Add snippets for /training/wearables/always-on (#691)

Author: ithinkihaveacat

wear/src/main/java/com/example/wear/snippets/alwayson/AmbientLifecycleObserver.kt

@@ -0,0 +1,64 @@
+/*
+ * Copyright 2025 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.example.wear.snippets.alwayson
+
+import android.os.Bundle
+import androidx.activity.ComponentActivity
+import androidx.wear.ambient.AmbientLifecycleObserver
+
+// [START android_wear_ongoing_activity_ambientlifecycleobserver]
+val ambientCallback = object : AmbientLifecycleObserver.AmbientLifecycleCallback {
+    override fun onEnterAmbient(ambientDetails: AmbientLifecycleObserver.AmbientDetails) {
+        // ... Called when moving from interactive mode into ambient mode.
+        // Adjust UI for low-power state: dim colors, hide non-essential elements.
+    }
+
+    override fun onExitAmbient() {
+        // ... Called when leaving ambient mode, back into interactive mode.
+        // Restore full UI.
+    }
+
+    override fun onUpdateAmbient() {
+        // ... Called by the system periodically (typically once per minute)
+        // to allow the app to update its display while in ambient mode.
+    }
+}
+// [END android_wear_ongoing_activity_ambientlifecycleobserver]
+
+class AmbientLifecycleActivity : ComponentActivity() {
+
+    private val activity = this // rename so the snippet reads better
+    // [START android_wear_ongoing_activity_ambientlifecycleobserver_oncreate]
+    private val ambientObserver = AmbientLifecycleObserver(activity, ambientCallback)
+
+    override fun onCreate(savedInstanceState: Bundle?) {
+        super.onCreate(savedInstanceState)
+        lifecycle.addObserver(ambientObserver)
+
+        // ...
+    }
+    // [END android_wear_ongoing_activity_ambientlifecycleobserver_oncreate]
+
+    // [START android_wear_ongoing_activity_ambientlifecycleobserver_ondestroy]
+    override fun onDestroy() {
+        super.onDestroy()
+        lifecycle.removeObserver(ambientObserver)
+
+        // ...
+    }
+    // [END android_wear_ongoing_activity_ambientlifecycleobserver_ondestroy]
+}