gatt: Add notify_value_changed() when writing attributes

When an attribute is written using btd_gatt_write_attribute() and device
is NULL, the core calls notify_value_changed() to send notification for
all devices which enabled notification.
gatt: Fix check for Notification/Indication PDU size

The minimal PDU size for Notification/Indication is 3 octets: opcode (1
octet) + handle (2 octets) + value parameter (can be 0 or more octets).
DEBUG: Add D-Bus method to write CCC

Write in CCC handle to enable/disable notification/indication from
remote.
DEBUG: Add value changed on reporter

Make "Link Loss Alert Level" characteristic to become notified. When
writing it, sends a value changed for core.

Add some debug messages when enabling notification.