Add boot detection in the NSE and use this to inform the user that there may be more notifications. (#5054)

* Add an initial implementation of the receivedWhileOfflineNotification.

* Only deliver a single receivedWhileOffline notification per boot and clarify the API.

* Add a 15-minute threshold for the receiveWhileOfflineNotification.

ElementX/Sources/Application/Settings/AppSettings.swift

@@ -14,7 +14,9 @@ import Foundation
 import SwiftUI
 
 /// Common settings between app and NSE
-protocol CommonSettingsProtocol {
+protocol CommonSettingsProtocol: AnyObject {
+    var lastNotificationBootTime: TimeInterval? { get set }
+    
     var logLevel: LogLevel { get }
     var traceLogPacks: Set<TraceLogPack> { get }
     var bugReportRageshakeURL: RemotePreference<RageshakeConfiguration> { get }
@@ -51,6 +53,7 @@ final class AppSettings {
         case enableNotifications
         case enableInAppNotifications
         case pusherProfileTag
+        case lastNotificationBootTime
         case logLevel
         case traceLogPacks
         case viewSourceEnabled
@@ -291,6 +294,10 @@ final class AppSettings {
     @UserPreference(key: UserDefaultsKeys.pusherProfileTag, storageType: .userDefaults(store))
     var pusherProfileTag: String?
     
+    /// The device's last boot time as recorded by the NSE.
+    @UserPreference(key: UserDefaultsKeys.lastNotificationBootTime, storageType: .userDefaults(store))
+    var lastNotificationBootTime: TimeInterval?
+    
     // MARK: - Logging
         
     @UserPreference(key: UserDefaultsKeys.logLevel, defaultValue: LogLevel.info, storageType: .userDefaults(store))