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/AppMediator.swift

@@ -17,7 +17,7 @@
 import UIKit
 
 class AppMediator: AppMediatorProtocol {
-    private let windowManager: WindowManagerProtocol
+    let windowManager: WindowManagerProtocol
     
     init(windowManager: WindowManagerProtocol) {
         self.windowManager = windowManager
@@ -28,7 +28,6 @@ class AppMediator: AppMediatorProtocol {
         UIApplication.shared
     }
 
-    @MainActor
     var appState: UIApplication.State {
         switch application.applicationState {
         case .active:
@@ -42,12 +41,8 @@ class AppMediator: AppMediatorProtocol {
         }
     }
     
-    var backgroundTimeRemaining: TimeInterval {
-        application.backgroundTimeRemaining
-    }
-    
-    func beginBackgroundTask(withName taskName: String?, expirationHandler handler: (() -> Void)?) -> UIBackgroundTaskIdentifier {
-        application.beginBackgroundTask(withName: taskName, expirationHandler: handler)
+    func beginBackgroundTask(expirationHandler handler: (() -> Void)?) -> UIBackgroundTaskIdentifier {
+        application.beginBackgroundTask(expirationHandler: handler)
     }
 
     func endBackgroundTask(_ identifier: UIBackgroundTaskIdentifier) {