For #4083: make findSessionById thread-safe

    val engine: Engine,

    delegate: Observable<SessionManager.Observer> = ObserverRegistry()

) : Observable<SessionManager.Observer> by delegate {

    // It's important that any access to `values` is synchronized;

    @GuardedBy("values")

    private val values = mutableListOf<Session>()

    @GuardedBy("values")

     * Finds and returns the session with the given id. Returns null if no matching session could be

     * found.

     */

    fun findSessionById(id: String): Session? = values.find { session -> session.id == id }

    fun findSessionById(id: String): Session? = synchronized(values) {

        values.find { session -> session.id == id }

    }

    /**

     * Informs this [SessionManager] that the OS is in low memory condition so it