Initial service implementation for using a PIN code. (#1912)

* Initial service implementation for using a PIN code.

* Tweak Danger for commit size

600-800 lines is perfectly normal for our PRs, up it to 1000.

ElementX/Sources/Services/AppLock/AppLockTimer.swift

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+//
+// 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 Foundation
+
+/// A timer that adds a grace-period to the app before locking it.
+class AppLockTimer {
+    /// The amount of time the app should remain unlocked for whilst backgrounded.
+    let gracePeriod: TimeInterval
+    
+    /// Whether the timer considers the app to be locked or not.
+    ///
+    /// Internally this value may be incorrect, always call `needsUnlock` to get the correct value.
+    private var isLocked = true
+    /// The date when the app was last backgrounded whilst in an unlocked state.
+    private var lastUnlockedBackground: Date?
+    
+    /// Creates a new timer.
+    /// - Parameter gracePeriod: The amount of time the app should remain unlocked for whilst backgrounded.
+    init(gracePeriod: TimeInterval) {
+        self.gracePeriod = 180
+    }
+    
+    /// Signals to the timer to track how long the app will be backgrounded for.
+    func applicationDidEnterBackground(date: Date = .now) {
+        // Only update the last background date if the app is currently unlocked.
+        guard !isLocked else { return }
+        lastUnlockedBackground = date
+    }
+    
+    /// Asks the timer to recompute the lock state on foregrounding.
+    func computeLockState(willEnterForegroundAt date: Date) -> Bool {
+        guard !isLocked, let lastUnlockedBackground else { return true }
+        
+        isLocked = date.timeIntervalSince(lastUnlockedBackground) >= gracePeriod
+        
+        // Don't allow changing the device's clock to unlock the app.
+        if date < lastUnlockedBackground {
+            isLocked = true
+        }
+        
+        return isLocked
+    }
+    
+    /// Registers a successful unlock with the timer.
+    func registerUnlock() {
+        isLocked = false
+    }
+}