ref(offline integration): Move event listener setup from constructor to setupOnce#2889

lobsterkatie merged 2 commits intogetsentry:mastergetsentry/sentry-javascript:masterfrom

piotr-cz:bugfix/offline-add-events-in-setupOncepiotr-cz/sentry-javascript:bugfix/offline-add-events-in-setupOnceCopy head branch name to clipboard