feat: Flexion channel build fixes

- Add google-services.json per-channel copy in SwitchChannel (Flexion has extra client)
- Add GooglePlay default google-services.json for fallback
- Switch Bundle Identifier per channel (Flexion → com.arkgame.ft.flexion)
- Fix FlexionSDKHelper missing BillingService import
- Fix Flexion AndroidManifest: add fullBackupContent override with flexion_backup_rules
- Add MainProcessName tools:replace to resolve gasdk/tuyoosdk manifest conflict
- Add billing:6.0.1 dependency for tuyoosdk AAR init compatibility

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

ExtraPluginCfgs/Android/flexion/mainTemplate.gradle

@@ -38,13 +38,13 @@ dependencies {
     api "io.reactivex:rxandroid:1.2.1"
     api "io.reactivex:rxjava:1.1.9"
 
-    // Google Play In-App Review 
+    // Google Play In-App Review
     implementation 'com.google.android.play:review:2.0.1'
+    // Google Play Billing — tuyoosdk AAR references BillingManager during init
+    implementation 'com.android.billingclient:billing:6.0.1'
     implementation 'net.aihelp:android-aihelp-aar:5.3.+'
     
     implementation 'com.google.android.gms:play-services-auth:20.2.0'
-    // NOTE: Google Play BillingClient NOT included — Flexion SDK provides its own billing wrapper
-    // implementation('com.android.billingclient:billing:6.0.1')
 
     // Flexion SDK (includes wrapped billing)
     implementation("com.flexionmobile:sdk:4.0.1")