Bluetooth: Add read_index_list management command

This patch implements the read_index_list command through which
userspace can get a list of current adapter indices.

Signed-off-by: Johan Hedberg <johan.hedberg@nokia.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>