[NUI][AT-SPI] Store AccessibilityAttributes in NUI (#4389)

[NUI][AT-SPI] Store AccessibilityAttributes in NUI (#4389)

Having a modifiable attribute dictionary in NUI removes the performance
penalty of having to go through interops on every modification. A new
override for GetAttributes in NUIViewAccessible will now collect the
attributes stored in NUI.

Co-authored-by: Artur Świgoń <a.swigon@samsung.com>