Exit notification timeouts immediately for disconnected devices

When a device disconnects it frees up the list of supported
notifications but is unable to free the timeout timer as well. This
change makes it so crashes won't occur when a timeout occurs for a
device that doesn't have any associated supported events.

Tag: #stability
Bug: 179741365
Test: atest BluetoothInstrumentationTests
Change-Id: I634717615a932a9d6f275bbf5b94ec5a9bd880e0