NMKeepAlive is a full API independent of NMActiveConnection. For good
reasons:

  - it moves complexity away from nm-active-connection.c

  - in the future, we can use NMKeepAlive also for NMSettingsConnection

As such, the user should also directly interact with NMKeepAlive,
instead of going through NMActiveConnection. For that to work, it
must be possible to get the owner of the NMKeepAlive instance,
which is kept alive.