Enable analytics only while authenticated

The one place where we should log out of PostHog and reset our analytics ID is when the user is logging out. This matches the behavior in Element Web and makes sense, I think, because logging out is essentially a request for the app to forget who you are. This means we should also start analytics at the point of logging in / reauthenticating.

I noticed while making this change that there was an unused branch in setClient, so I cleaned it up rather than making myself update it.

src/analytics/PosthogAnalytics.ts

@@ -13,6 +13,7 @@ import posthog, {
 import { logger } from "matrix-js-sdk/src/logger";
 import { type MatrixClient } from "matrix-js-sdk/src/matrix";
 import { Buffer } from "buffer";
+import { type Subscription } from "rxjs";
 
 import { widget } from "../widget";
 import {
@@ -101,6 +102,7 @@ export class PosthogAnalytics {
   private anonymity = Anonymity.Disabled;
   private platformSuperProperties = {};
   private registrationType: RegistrationType = RegistrationType.Guest;
+  private optInListener: Subscription | null = null;
 
   public static hasInstance(): boolean {
     return Boolean(this.internalInstance);
@@ -146,7 +148,6 @@ export class PosthogAnalytics {
       );
       this.enabled = false;
     }
-    this.startListeningToSettingsChanges(); // Triggers maybeIdentifyUser
   }
 
   private sanitizeProperties = (
@@ -328,6 +329,8 @@ export class PosthogAnalytics {
     if (this.enabled) {
       this.posthog.reset();
     }
+    this.optInListener?.unsubscribe();
+    this.optInListener = null;
     this.setAnonymity(Anonymity.Disabled);
   }
 
@@ -406,7 +409,7 @@ export class PosthogAnalytics {
     }
   }
 
-  private startListeningToSettingsChanges(): void {
+  public startListeningToSettingsChanges(): void {
     // Listen to account data changes from sync so we can observe changes to relevant flags and update.
     // This is called -
     //  * On page load, when the account data is first received by sync
@@ -415,7 +418,7 @@ export class PosthogAnalytics {
     //  * When the user changes their preferences on this device
     // Note that for new accounts, pseudonymousAnalyticsOptIn won't be set, so updateAnonymityFromSettings
     // won't be called (i.e. this.anonymity will be left as the default, until the setting changes)
-    optInAnalytics.value$.subscribe((optIn) => {
+    this.optInListener ??= optInAnalytics.value$.subscribe((optIn) => {
       this.setAnonymity(optIn ? Anonymity.Pseudonymous : Anonymity.Disabled);
       this.maybeIdentifyUser().catch(() =>
         logger.log("Could not identify user"),