android/gatt: Add read_cb for GATT Server
android/gatt: Add read_cb for GATT Server

This patch attach read callback to descriptors and characteristics
registered by Android app.

pkarasev3/bluez,pkarasev3/bluez,mapfau/bluez,pkarasev3/bluez,ComputeCycles/bluez,silent-snowman/bluez,mapfau/bluez,ComputeCycles/bluez,pkarasev3/bluez,mapfau/bluez,ComputeCycles/bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,ComputeCycles/bluez,silent-snowman/bluez,pstglia/external-bluetooth-bluez,silent-snowman/bluez,mapfau/bluez,pstglia/external-bluetooth-bluez,pstglia/external-bluetooth-bluez