HIDD: Auto-plug and accept incoming connections

Linux, Windows, and OSX try to connect with HID devices during
pairing. We should auto-plug and accept incoming connections if
there is currently no device connected.

In case of Windows, if it fails to connect the first time right
after pairing, all future connections are guaranteed to fail:
hangs in the "response pending" state at L2CAP connection, right
after SECURITY_COMPLETE, looks like a bug in Windows stack.

Since we always have a plugged, "in_use" device in registered
state, we won't be able to accept a new incoming connection,
unless we don't have any other paired device at all. This check
should be removed to allow smoother pairing experience and fix
Windows compatibility.

Bug: 66940516
Test: manual, with a test app
Change-Id: Ie6ca639cb120b52f59880fadb4d3654a095664d6