Fix compilation error

Reviewed-on: community/device-mgt-plugins#72

components/mobile-plugins/android-plugin/io.entgra.device.mgt.plugins.mobile.android.api/src/test/java/io/entgra/device/mgt/plugins/mobile/android/api/mocks/DeviceManagementProviderServiceMock.java

@@ -906,4 +906,9 @@ public class DeviceManagementProviderServiceMock implements DeviceManagementProv
     public OperationDTO getOperationDetailsById(int i) throws OperationManagementException {
         return null;
     }
+
+    @Override
+    public PaginationResult getDevicesNotInGroup(PaginationRequest paginationRequest, boolean b) throws DeviceManagementException {
+        return null;
+    }
 }