[Bluetooth] Use a more unique bus name for connections

When attaching Bluetooth low-level connections to the bus, the bus name
is constructed from the remote address since at that time the connection
handle is not assigned yet. This has worked so far, but also caused a
lot of troubles. It is better to postpone the creation of the sysfs
entry to the time when the connection actually has been established
and then use its connection handle as unique identifier.

This also fixes the case where two different adapters try to connect
to the same remote device.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>