Add support for Face ID/Touch ID to app lock. (#1966)

* Fix biometrics with low grace period and backgrounding before unlocked.

* Trigger permissions alert when enabling Face ID.

ElementX/Sources/Services/Keychain/KeychainControllerProtocol.swift

@@ -44,4 +44,12 @@ protocol KeychainControllerProtocol: ClientSessionDelegate {
     func pinCode() -> String?
     /// Removes the App Lock PIN code.
     func removePINCode()
+    /// Whether or not PIN code biometric state has been set.
+    func containsPINCodeBiometricState() -> Bool
+    /// Sets the PIN code biometric state for App Lock.
+    func setPINCodeBiometricState(_ state: Data) throws
+    /// The PIN code biometric state required to use Touch/Face ID to unlock the app.
+    func pinCodeBiometricState() -> Data?
+    /// Removes the App Lock PIN code biometric state.
+    func removePINCodeBiometricState()
 }