Add GATT_CONNECTION_TIMEOUT and change connection timeout flow

Replace GATT_ERROR being returned to the developer when GATT connection
times out after 30 seconds, with new status code
GATT_CONNECTION_TIMEOUT.

Bug: 201571634
Bug: 320574336
Test: atest GattClientTest
Change-Id: I09e7fe32b67b40f5281f1f8cddfd2863bee2993e