android/gatt: Add test command for sign write
android/gatt: Add test command for sign write

This is needed for some PTS test cases.

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