core: signal parent-active only when the parent AC is activated
The parent-active signal is used by the manager to determine when the parent active-connection is ready and a connection that depend on it can proceed.
The AC state could transition from ACTIVATING directly to DEACTIVATING; in such case we should not emit the signal but instead just stop watching the parent AC.
Fixes: 6e382ea9 ('active-connection: add parent active connection tracking') https://bugzilla.redhat.com/show_bug.cgi?id=1778073