[Tizen] Provide GetActiveWindow interface

[Tizen] Provide GetActiveWindow interface

If there are two AT clients, AT server does not send 'activate' signal
for the second AT client. Because AT server send the 'activate' signal
when the 'IsEnabled' status was set to be TRUE.

AT client is able to search the 'activate' window using the desktop and
its children - applications, but some times it is very slow. Probably..
It is caused by 'throttle' feature of EFL application.

We need to keep the 'activate' window information for the second client.
The at-spi2-registryd will keep the 'activate' window information.
The AT client gets the information using atspi_get_activate_window.

Change-Id: Ia5f22465b10a46411768866f245aa8309b2cc013