Improve adv_data transfer between contexts.

- Drops some unused members from btgatt_multi_adv_inst_cb.
- Flatten tBTA_BLE_ADV_DATA to only require one allocation.
- Copy all of tBTA_BLE_ADV_DATA contents to bt_workqueue_thread.

Bug: 23042223
Change-Id: I5f4d25d5cdd39731a2dfe64394928b7fb77c7c7c