@NonNull
	protected ValueChangedCallback setWriteCallback(@Nullable final BluetoothGattCharacteristic serverCharacteristic) {
		return requestHandler.getValueChangedCallback(serverCharacteristic);
	}
Returns the callback that is registered for value changes (write command or write request
initiated by the remote device) of given characteristic.
<p>
To remove the callback, call
{@link #removeWriteCallback(BluetoothGattCharacteristic)}.

@param serverCharacteristic characteristic to bind the callback with. If null, the returned
                      	   callback will not be null, but will not be used.
@return The callback.
ble/src/main/java/no/nordicsemi/android/ble/BleManager.java