[AT-SPI] remove isRoot param from Accessible::Get
The Accessibility::Accessible::Get for a window could be called
before getting accessible to add window accessible.
So far, we thought that the next part creates an accessible for
window for the first time.
accessible = Accessibility::Accessible::Get(rootLayer, true);
bridge->AddTopLevelWindow(accessible);
However, there is a case where it is created before this part
caused by following symbol
CSharp_Dali_Toolkit_DevelControl_GetAccessibilityStates+0x144
In this case, isRoot is set to `false`. Then window will have
incorrect accessible information.
For more information, please refer to the following.
https://github.sec.samsung.net/tizen/atspi/issues/60
Change-Id: Id2d27f35426e72b67986f132f0c77979016b4252