kdbus: move dbuspolicy init to bus_register_kdbus

kdbus: move dbuspolicy init to bus_register_kdbus

This changes the moment of checking if user is allowed to connect to
the bus. Now, it reflects standard process a bit more.

The standard process is:
- open fd (e.g. socket);
- check authentication, if needed and possible;
- connect to the bus (say hello).

In kdbus, we have only:
- open kdbus fd;
- connect to the bus (ioctl KDBUS_CMD_HELLO).
Calling libdbuspolicy for authentication fits between the two.

Additionally, and most importantly, this is required to share
the connection between libdbus and libdbuspolicy in the future.

Change-Id: Id6fe1dbc1cdc6ec774316e13fe5d60d862949476