Bluetooth: Add discovering event to the Management interface

This patch adds a new event to the Management interface to track when
local adapters are discovering remote devices. For now this only tracks
BR/EDR discovery procedures.

Signed-off-by: Johan Hedberg <johan.hedberg@nokia.com>
Signed-off-by: Anderson Briglia <anderson.briglia@openbossa.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>