Cleanup following the AppMediator introduction (#2723)

- stop using multiple background task, the appCoordinator sync one is enough for the whole app
- move the AppMeditor to the MainActor
- expose the WindowManager through the AppMediator
- hide sensitive WindowManager API behind a different protocol
- remove the now unnecessary `BackgroundTaskService`

ElementX/Sources/Application/AppCoordinatorProtocol.swift

@@ -17,6 +17,6 @@
 import Foundation
 
 protocol AppCoordinatorProtocol: CoordinatorProtocol {
-    var windowManager: WindowManagerProtocol { get }
+    var windowManager: SecureWindowManagerProtocol { get }
     @discardableResult func handleDeepLink(_ url: URL, isExternalURL: Bool) -> Bool
 }