GATT: Unregister notifications when disconnecting

Currently, if we receive notification after disconnection, but before
closing the client, stack would notify upper layers that connection is
avaliable, and send notifications to Java.

This can be troublesome i.e. for Heart Rate Monitors, that send
notifications very often, and can prevent apps from closing properly.

Bug: 231818024
Test: connect to HRM with more than one application, close/disconnect
multiple times

Change-Id: Icb68c9107bb9dbd54ca0e48d4cabea5da143055e