Invalidate biometric on change (#1026)

* Initial working version for Android

* Add a fallback for when upgrading from older app version.

* Ensure biometric validity is re-checked on focus

* Only setup biometric integrity key if biometric is turned on.

* Fix styling according to comments

* Fallback for Android 5.

* Improve comment

* Add boilerplate for iOS

* Change BiometricService to public

* Untested iOS implementation.

* Convert IBiometricService to async. Fix code style for iOS.

* Base64 NSData.

* Review comments for Android BiometricService.

* Rename methods in BiometricService to append Async

* Ensure we wait for async SetupBiometricAsync.

* Update BiometricService.cs

Co-authored-by: Kyle Spearrin <kspearrin@users.noreply.github.com>

src/iOS.Core/Services/BiometricService.cs

@@ -0,0 +1,62 @@
+using System.Threading.Tasks;
+using Bit.Core.Abstractions;
+using Foundation;
+using LocalAuthentication;
+
+namespace Bit.iOS.Core.Services
+{
+    public class BiometricService : IBiometricService
+    {
+        private IStorageService _storageService;
+
+        public BiometricService(IStorageService storageService)
+        {
+            _storageService = storageService;
+        }
+
+        public async Task<bool> SetupBiometricAsync()
+        {
+            var state = GetState();
+            await _storageService.SaveAsync("biometricState", ToBase64(state));
+
+            return true;
+        }
+
+        public async Task<bool> ValidateIntegrityAsync()
+        {
+            var oldState = await _storageService.GetAsync<string>("biometricState");
+            if (oldState == null)
+            {
+                // Fallback for upgraded devices
+                await SetupBiometricAsync();
+
+                return true;
+            }
+            else
+            {
+                var state = GetState();
+
+                return FromBase64(oldState) == state;
+            }
+        }
+
+        private NSData GetState()
+        {
+            var context = new LAContext();
+            context.CanEvaluatePolicy(LAPolicy.DeviceOwnerAuthenticationWithBiometrics, out _);
+
+            return context.EvaluatedPolicyDomainState;
+        }
+
+        private string ToBase64(NSData data)
+        {
+            return System.Convert.ToBase64String(data.ToArray());
+        }
+
+        private NSData FromBase64(string data)
+        {
+            var bytes = System.Convert.FromBase64String(data);
+            return NSData.FromArray(bytes);
+        }
+    }
+}