message: Fix stickerpicker PersistedElement usage
  Earlier PR changed the prop to persistKey in PersistedElement but
  containerId here, so this was ending up as 'undefined'.
diff: @@ -215,7 +215,7 @@ export default class Stickerpicker extends React.Component {
  width: this.popoverWidth,
  }}
  >
  - <PersistedElement containerId="mx_persisted_stickerPicker" style={{zIndex: STICKERPICKER_Z_INDEX}}>
  + <PersistedElement persistKey="stickerPicker" style={{zIndex: STICKERPICKER_Z_INDEX}}>
  <AppTile
  collectWidgetMessaging={this._collectWidgetMessaging}
  id={stickerpickerWidget.id}
  