GATT: Disable 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 sned 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
Merged-In: I1e9ca7ba6379322f7321b77a2ea92c96c799a452
Change-Id: I1e9ca7ba6379322f7321b77a2ea92c96c799a452
(cherry picked from commit 666f247f)