[AT-SPI] Emit fewer BoundsChanged events for Window

[AT-SPI] Emit fewer BoundsChanged events for Window

1. Check whether the window geometry has really changed before emitting events.

2. Also check Accessibility::IsUp() beforehand. This condition is also checked
internally by EmitBoundsChanged(), but too much of the Accessibility D-Bus
infrastructure will have to be initialized before reaching that point in the
code, and this initialization is not always needed, e.g. during early
application initialization.

Change-Id: I9c3898812380a4d98a0adcbe56445e6af31bf465