Refactor tGATT_BG_CONN_DEV

* Use list for keeping the devices instead of fixes array, get rid of
  the in_use field.
* Use unordered_set instead of array to keep gatt_if of apps interested
  in specific device.
* the in_use flag was never cleared before, this had the potential to
  fill the whole array when multiple devices were addred/removed to the
  whitelist. This issue is fixed now.

Test: compilation test
Change-Id: Id87c2ae9c5b0ce0887a4fa9be50e34d646d22a13