gatt: Introduce helper for reading attribute values

This patch adds the btd_gatt_read_attribute() helper which reads the
value from an attribute.
gatt: Introduce helper for writing in attribute values

This patch adds the btd_gatt_write_attribute() helper which writes in
the value from an attribute.
gatt: Introduce helper for registering a notifier

This patch adds the btd_gatt_add_notifier() helper which registers a
notifier to be warned when the specified attribute's value changes.
gatt: Introduce helper for unregistering a notifier

This patch adds the btd_gatt_remove_notifier() helper which unregisters
a previously registered notifier for an specific attribute.
lib: Move GATT UUIDs to uuid.h
proximity: Add new Immediate Alert Service files

This patch adds new files to implement Immediate Alert Service using
the new attribute API. Files using the old GATT API will be removed
later. The proposed solution doesn't handle multiple local Bluetooth
adapters yet.
proximity: Add Immediate Alert Service

This patch adds the service and characteristic declaration of the
Immediate Alert Service using the new attribute API.
proximity: Add remove Immediate Alert Service

This patch removes the Immediate Alert Service declaration when the
adapter is removed. The current solution doesn't handle multiple local
Bluetooth adapters.
proximity: Remove unneeded control logic

This patch removes unneeded list of adapters and devices. device_remove
callback of the btd_profile always gets called when the device or the
adapter related to the device is removed.
scanparam: Remove unneeded header include
alert: Remove unneeded header include
gatt: Add listen socket for GATT server

This patch adds the initial code for the new GATT server integrated with
the Generic Attribute API. The legacy code will stay for a while.
gdbus: Add g_dbus_get_flags()
gatt: Enable new server if experimental is set

The current GATT server conflicts with the new Attribute Server that is
being developed. This patch enables the new Attribute Server is
Experimental option is informed.
gatt: Add GAP service

This patch adds GAP service definition and it's attributes in the new
Attribute server.
gatt: Add GATT service

This patch adds GATT service definition and it's attributes in the new
Attribute Server.
gatt: Register ATT commands/events handler

This patch registers the glib channel watch to manage ATT commands and
events.
gatt: Add reading ATT incoming data

This patch adds the code to read the incoming ATT traffic and for now
it replies "Request Not Supported" for all requests.
gatt: Add attribute parameter to the read/write callbacks
WIP: gatt: Introduce attribute read cb for GATT client

This patch adds the callback function to be called when a GATT client
application reads the attribute value. Its implementation sends an ATT
Request Value.

Currently there is no error handling.