Don't restart the sync service if the client is shutting down (#7472)

* Don't restart the sync service if the client is shutting down

This was causing issues when clearing the cache:

1. The client would ask the SDK to clear the caches.
2. The SDK would stop the sync service, clear the caches and set the `pos` value of sliding sync to 0 so the HS returns all the required state the next time the client is used.
3. However, between the sync service being stopped with the `pos=0` change and the client being destroyed, there was enough time for the `SyncOrchestrator` to decide the sync service was stopped for no good reason since there is network connectivity and the app is on foreground, so it restarted the sync, with `pos=0`.
4. This value was persisted, and the next time the new client did an 'initial sync' it had a `pos != 0`, which the HS uses to avoid returning some required state it already fetched.
5. The room list now lacks state events for lots of rooms and room names, avatars, members, etc. are now missing.

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Co-authored-by: Benoit Marty <benoitm@element.io>

libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/MatrixClient.kt

@@ -136,6 +136,9 @@ interface MatrixClient : UrlContentFetcher {
     /** Scans media content before it is displayed, or `null` when no content scanner is configured for this session. */
     val contentScanner: ContentScanner?
 
+    /** Whether the session is in the process of shutting down, either because it is being logged out or its cache is being cleared. */
+    val isShuttingDown: Boolean
+
     /**
      * Returns the room with the given id if the user has joined it, along with its live [Timeline].
      * A new instance is created on each call and the caller owns it, so it must be closed once no longer needed.