BTA GATT operation queue

Currently, HID BTA app have own GATT queue for scheduling reads/writes
during device setup. Such queue will also be needed for Hearing Aid.
This patch makes the GATT queue code generic and clean it up.

Test: manual, bond HID device
Bug: 69623109
Change-Id: Ic893d630ab5154f7c1875748f3e275cbad02a7b2