Fix: prevent unintended app launch with Firebase + audio_service

Apply changes from PR ryanheise#1011 by @anocean2 (Andrey Kupriev).
This fixes the issue where Firebase messages cause the app to launch
when it shouldn't by adding a startup guard to AudioServiceActivity.

Changes:
- Add armStartup flag to AudioServicePlugin
- getFlutterEngine() returns null when not armed
- AudioServiceActivity calls setArmStartup(true) before getting engine

Original PR: ryanheise#1011
Co-Authored-By: Andrey Kupriev <kuprievao@intech.rshb.ru>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: 3 people

audio_service/android/src/main/java/com/ryanheise/audioservice/AudioServiceActivity.java

@@ -10,6 +10,7 @@
 public class AudioServiceActivity extends FlutterActivity {
     @Override
     public FlutterEngine provideFlutterEngine(@NonNull Context context) {
+        AudioServicePlugin.setArmStartup(true);
         return AudioServicePlugin.getFlutterEngine(context);
     }
 }

audio_service/android/src/main/java/com/ryanheise/audioservice/AudioServicePlugin.java

@@ -59,6 +59,10 @@
  * AudioservicePlugin
  */
 public class AudioServicePlugin implements FlutterPlugin, ActivityAware {
+    private static boolean armStartup = false;
+    public static synchronized void setArmStartup(boolean enabled) {
+        armStartup = enabled;
+    }
     private static String flutterEngineId = "audio_service_engine";
     /** Must be called BEFORE any FlutterEngine is created. e.g. in Application class. */
     public static void setFlutterEngineId(String id) {
@@ -67,7 +71,11 @@ public static void setFlutterEngineId(String id) {
     public static String getFlutterEngineId() {
         return flutterEngineId;
     }
+
     public static synchronized FlutterEngine getFlutterEngine(Context context) {
+        if (armStartup == false) {
+            return null;
+        }
         FlutterEngine flutterEngine = FlutterEngineCache.getInstance().get(flutterEngineId);
         if (flutterEngine == null) {
             // XXX: The constructor triggers onAttachedToEngine so this variable doesn't help us.