Remove extra string/attributed string creation in accessibility props

[Adlai-Holler]

Motivation

We currently create attributed & non-attributed copies of accessibility hint, label, and value whenever you set them. We don't need to – UIKit will bridge between the two worlds for us. Creating attributed strings is actually pretty expensive. So we just update our flags and set whichever one was set most recently.

Changes

• Remove nonattributed <-> attributed duplication in accessibility properties.
• In the getters (only used before-node-load), generate the corresponding value on demand.
  • Will almost never be called.
• Always update "changed" flags to ensure the latest value is applied in the case that the view was given a different value outside of ASPendingState.