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`

UnitTests/Sources/MediaProvider/MediaProviderTests.swift

@@ -22,15 +22,13 @@ import XCTest
 final class MediaProviderTests: XCTestCase {
     private let mediaLoader = MockMediaLoader()
     private var imageCache: MockImageCache!
-    private var backgroundTaskService = MockBackgroundTaskService()
     
     var mediaProvider: MediaProvider!
     
     override func setUp() {
         imageCache = MockImageCache(name: "Test")
         mediaProvider = MediaProvider(mediaLoader: mediaLoader,
-                                      imageCache: imageCache,
-                                      backgroundTaskService: backgroundTaskService)
+                                      imageCache: imageCache)
     }
     
     func test_whenImageFromSourceWithSourceNil_nilReturned() throws {

UnitTests/Sources/MediaProvider/MockBackgroundTaskService.swift

@@ -1,23 +0,0 @@
-//
-// Copyright 2022 New Vector Ltd
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-//
-// http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-//
-@testable import ElementX
-import Foundation
-
-class MockBackgroundTaskService: BackgroundTaskServiceProtocol {
-    func startBackgroundTask(withName name: String, isReusable: Bool, expirationHandler: (() -> Void)?) -> ElementX.BackgroundTaskProtocol? {
-        nil
-    }
-}