[AT-SPI] Fix not working screen reader when rerunning it

[AT-SPI] Fix not working screen reader when rerunning it

[Reproduce Steps]
 1. Screen Reader ON
 2. Run any application
 3. Turn Screen Reader OFF
 4. Turn Screen Reader ON

- It is caused by NULL windowAccessible which is returned
 by following function used by EmitActivate()
 Dali::Accessibility::Accessible* GetWindowAccessible(Dali::Window window)

- Connected Enabled / Disabled signals to Window
 so that Window can add and remove itself from bridge
 when the state of bridge changes.

Change-Id: I3bdebedd7bf4f9017d24b72999412a0f70cfd24b
Signed-off-by: Seoyeon Kim <seoyeon2.kim@samsung.com>