Periodically update sensors from local push extension (#2138)

## Summary
When the Local Push extension is running (when we're on Wi-Fi in an SSID marked as 'Internal' and location is set to Always), we're given a persistent opportunity to do work. When we're running, we now update sensors on a regular interval.

## Link to pull request in Documentation repository
Documentation: home-assistant/companion.home-assistant#753

## Any other notes
Updating the metadata/sensors every 5 minutes (by default) should have minimal, if any, battery implications. This follows the same setting as the "Periodic" timer which we use for foreground updates.

Sources/Shared/API/HAAPI.swift

@@ -136,10 +136,11 @@ public class HomeAssistantAPI {
         case cold
         case warm
         case periodic
+        case background
 
         var updateSensorTrigger: LocationUpdateTrigger {
             switch self {
-            case .cold, .warm:
+            case .cold, .warm, .background:
                 return .Launch
             case .periodic:
                 return .Periodic