Properly disconnect GATT connection during noisy scans

Properly disconnect GATT connection during noisy scans

This patch fixes an issue that is reproducible in highly noisy environments
(massive deployment of BLE beacons), through repeated connect/disconnect
attempts on a remote peripheral while scanning for beacons in the background.
The state machine in bta/gatt has a special control flow for handling disconnect
requests during discovery, which in this case failed to resolve the original
request by issuing an HCI_Disconnect command. This is now fixed by always
explicitly triggering the connection close sequence once the discovery state has
been cleaned up.

This patch also includes a fix for a crash that occurred as a side-effect of the
scenario described above.

Bug: 22350508
Change-Id: Ie9cbd3c8f54239b142bfb8dde80d9581ae70ed43