android/gatt: Add write_cb to GATT server
android/gatt: Add write_cb to GATT server

Write callback is now attached to characteristics and descriptors
registered by Android app.

Transaction id is generated and stored in gatt_app together with
att_opcode

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