DeviceManager: Remember auto-detected devices.

The current implementation "forgets" auto-detected devices when
they've been disconnected or Creator has been closed.
This has the drawback that potential changes by users to the connection
parameters cannot become persistent. This is overcome by introducing a
device fingerprint and keeping removed auto-connected devices in a
special list from which they can later be retrieved, typically when the
device gets connected again.

Change-Id: I98cfd25c677e6a2a46891a0facf3a28f0a0c3465
Reviewed-by: Nikolai Kosjar <nikolai.kosjar@nokia.com>