test: Add Link Loss service to gatt-service

This patch adds an example of Link Loss Service implementation. Alert
level characteristic is implemented following the mandatory operations
that need to be supported by a Link Loss Service: Read Request and Write
Request.
gatt: Add helper for removing GATT services

This patch adds the btd_gatt_remove_service() helper which removes a GATT
primary (or secondary) Service declaration from the local attribute database.
gatt-dbus: Add multiple services per application

This patch allows one application to register more than one GATT
service.
gatt: Introduce helper for adding descriptors

This patch adds the btd_gatt_add_char_desc() helper which adds a
characteristic descriptor to local attribute database.
gatt: Add support to find info request

This patch adds support to ATT Find Information Requests, this command
allows implementing the GATT sub-procedure to discover all characteristic
descriptors's Attribute Handles and Attribute Types within a characteristic
definition when only the characteristic handle range is known.
gatt-service: Register test descriptors interfaces