fix: android background backups (#21795)

* upload using dart client

* add connectivity api

* respect backup network setting

* comment as to why we need to wait for setForegroundAsync call

* log assets skipped due to network constraint

* dynamic spawning -> false

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Co-authored-by: shenlong-tanwen <139912620+shalong-tanwen@users.noreply.github.com>
Co-authored-by: Alex <alex.tran1502@gmail.com>

mobile/pigeon/background_worker_api.dart

@@ -24,6 +24,8 @@ abstract class BackgroundWorkerBgHostApi {
   // required platform channels to notify the native side to start the background upload
   void onInitialized();
 
+  void showNotification(String title, String content);
+
   // Called from the background flutter engine to request the native side to cleanup
   void close();
 }