Enable BLE background connection when stack is started

Remove variable btm_cb.ble_ctr_cb.bg_conn_type. We can enable the
background mode when the Bluetooth stack is started and don't have
to wait until the first background connection request from HID
Device/Hearing Aid/GATT. As soon as we add the first device to white
list, the stack will send HCI Create Connection (White List) command.

Test: manual. Bond to a BLE mouse and turn off/on Bluetooth. Also test
with initiating another direct/white list connection while another white
list connection is pending
Bug: 111562702
Bug: 112827989
Change-Id: I82a476489d181d17aa1c1da8202df1be76928d77