android/client: Add GATT server callbacks code
android/client: Add GATT server callbacks code

This patch adds callbacks code for GATT server interface.

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