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/build.gradle

@@ -11,6 +11,7 @@ android {
     ndkVersion = flutter.ndkVersion
 
     compileOptions {
+        coreLibraryDesugaringEnabled true
         sourceCompatibility = JavaVersion.VERSION_11
         targetCompatibility = JavaVersion.VERSION_11
     }
@@ -21,7 +22,7 @@ android {
 
     defaultConfig {
         applicationId = "io.cloudhosting.twp.twp_softphone"
-        minSdkVersion 24
+        minSdkVersion 26
         targetSdk = flutter.targetSdkVersion
         versionCode = flutter.versionCode
         versionName = flutter.versionName
@@ -42,6 +43,7 @@ flutter {
 }
 
 dependencies {
+    coreLibraryDesugaring 'com.android.tools:desugar_jdk_libs:2.1.4'
     implementation platform('com.google.firebase:firebase-bom:33.0.0')
     implementation 'com.google.firebase:firebase-messaging'
 }