fixing concurrent dispatch crash by using copy on write set

- whilst closing the session, we iterate through all the listeners calling onSessionStopped however some implementations also use this callback to remove themselves from the listeners, causing the concurrent modification crash

changelog.d/5821.bugfix

@@ -0,0 +1 @@
+Fixes concurrent modification crash when signing out or launching the app

matrix-sdk-android/src/main/java/org/matrix/android/sdk/internal/session/SessionListeners.kt

@@ -19,12 +19,13 @@ package org.matrix.android.sdk.internal.session
 import org.matrix.android.sdk.api.extensions.tryOrNull
 import org.matrix.android.sdk.api.session.Session
 import timber.log.Timber
+import java.util.concurrent.CopyOnWriteArraySet
 import javax.inject.Inject
 
 @SessionScope
 internal class SessionListeners @Inject constructor() {
 
-    private val listeners = mutableSetOf<Session.Listener>()
+    private val listeners = CopyOnWriteArraySet<Session.Listener>()
 
     fun addListener(listener: Session.Listener) {
         synchronized(listeners) {