Fix focus sensor updating in iOS 15 RC (#1842)

## Summary
Fixes the insta-updating of the focus sensor on iOS 15.

## Any other notes
`INFocusStatusCenter.default.focusStatus` always returns isFocused=false in the extension, but the intent provides the correct focus status for each update -- so we need to cache this value in the extension, and not trust the focus center state.

Sources/Shared/API/Webhook/Sensors/FocusSensor.swift

@@ -4,6 +4,7 @@ import PromiseKit
 final class FocusSensor: SensorProvider {
     public enum FocusError: Error, Equatable {
         case unauthorized
+        case unavailable
     }
 
     let request: SensorProviderRequest
@@ -12,6 +13,10 @@ final class FocusSensor: SensorProvider {
     }
 
     func sensors() -> Promise<[WebhookSensor]> {
+        guard Current.focusStatus.isAvailable() else {
+            return .init(error: FocusError.unavailable)
+        }
+
         guard Current.focusStatus.authorizationStatus() == .authorized else {
             return .init(error: FocusError.unauthorized)
         }