Introduce an AppMediator which will:

- combine multiple services to handle app wide requests
- remove direct UIApplication access from everywhere
- fix #2703 by combining UIApplication.applicationState with the WindowManager's main window appearance
- leave the door open for more app control in the future (especiall for the mac)

ElementX/Sources/Mocks/AppMediatorMock.swift

@@ -0,0 +1,61 @@
+//
+// Copyright 2023 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.
+//
+
+import UIKit
+
+extension AppMediatorMock {
+    static var `default`: AppMediatorProtocol {
+        AppMediatorMock(withState: .active,
+                        backgroundTimeRemaining: 10,
+                        allowTasks: true)
+    }
+    
+    static var mockBroken: AppMediatorProtocol {
+        AppMediatorMock(withState: .inactive,
+                        backgroundTimeRemaining: 0,
+                        allowTasks: false)
+    }
+    
+    static var mockAboutToSuspend: AppMediatorProtocol {
+        AppMediatorMock(withState: .background,
+                        backgroundTimeRemaining: 2,
+                        allowTasks: false)
+    }
+    
+    private static var bgTaskIdentifier = 0
+
+    convenience init(withState applicationState: UIApplication.State,
+                     backgroundTimeRemaining: TimeInterval,
+                     allowTasks: Bool) {
+        self.init()
+        
+        underlyingAppState = applicationState
+        underlyingBackgroundTimeRemaining = backgroundTimeRemaining
+        
+        beginBackgroundTaskWithNameExpirationHandlerClosure = { _, handler in
+            guard allowTasks else {
+                return .invalid
+            }
+            Self.bgTaskIdentifier += 1
+
+            let identifier = UIBackgroundTaskIdentifier(rawValue: Self.bgTaskIdentifier)
+            DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
+                handler?()
+            }
+            return identifier
+        }
+    }
+}