Fix Android build: bump AGP to 8.7, minSdk to 26, enable desugaring

- AGP 8.1.0 -> 8.7.0 (Flutter 3.41 minimum)
- Kotlin 1.8.22 -> 2.1.0
- minSdkVersion 24 -> 26 (twilio_voice requirement)
- Enable coreLibraryDesugaring for flutter_local_notifications
- Add placeholder google-services.json for build

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

mobile/android/app/google-services.json

@@ -0,0 +1,29 @@
+{
+  "project_info": {
+    "project_number": "000000000000",
+    "project_id": "twp-softphone-placeholder",
+    "storage_bucket": "twp-softphone-placeholder.appspot.com"
+  },
+  "client": [
+    {
+      "client_info": {
+        "mobilesdk_app_id": "1:000000000000:android:0000000000000000",
+        "android_client_info": {
+          "package_name": "io.cloudhosting.twp.twp_softphone"
+        }
+      },
+      "oauth_client": [],
+      "api_key": [
+        {
+          "current_key": "PLACEHOLDER_API_KEY"
+        }
+      ],
+      "services": {
+        "appinvite_service": {
+          "other_platform_oauth_client": []
+        }
+      }
+    }
+  ],
+  "configuration_version": "1"
+}