Make AdvertiseManager shutdown gracefully

If the Bluetooth stack is shutdown while AdvertiseManager has a pending HCI
operation being executed, the callback to that operation might be
executed after the manager was deleted. In order to fix any potential
issues, always use a weak pointer to the advertisement manager. This will
prevent the callbacks from getting executed.

In case of callbacks that are not executed on the AdvertiseManager
instance, one can check if the weak pointer is still valid, and properly
handle the stack shutdown.

Test: BleAdvertisingManagerTest:test_cleanup_during_execution
Change-Id: I8d33de4f64267a880880082c5498375c083addef