Preserve unset accessibilityState.selected in C++ props (#58599)

[crazeface]

Summary:

AccessibilityState::selected was a plain bool defaulting to false, so the
native side could not tell a component that is selectable but currently
unselected (accessibilityState={{selected: false}}) from one that is not
selectable at all (accessibilityState={{}}). Both arrived as false. The JS
type is already selected?: ?boolean, so this is the bridge discarding a value
the public API accepts.

Make selected a std::optional<bool> defaulting to std::nullopt.

JS accessibilityState native selected (before -> after)
{} false -> undefined
{selected: false} false -> false
{selected: true} true -> true

This aligns the representation with ARIA, which accessibilityState mirrors:
the bool / tri-state split now tracks which ARIA attributes admit an undefined
value.

field ARIA value type admits undefined representation
disabled boolean no bool
busy boolean no bool
selected boolean yes std::optional (changed)
expanded boolean yes std::optional
checked tristate yes CheckedState (None = unset)

That is also why disabled and busy stay plain bool: ARIA gives them no
undefined value, so there is no unset state to preserve. expanded was made
optional for this same reason in
#40881 and checked has always
carried a None; selected was the outlier.

Nor is "unset" merely "absent" for this attribute. testing-library/dom
computes it as boolean | undefined, documented "false/true if (not)selected,
undefined if not selectable" -- the same shape, with the same meaning, that
this change introduces.

Host platforms need the distinction: on Windows a selectable component must
implement ISelectionItemProvider so UIA can report selection state, and with
the old representation every component carrying an accessibilityState looked
selectable.

iOS and Android rendering is unchanged. Trait derivation coalesces the optional
with value_or(false), and the Android serializer omits the key when the value
is unset, which BaseViewManager#setViewState already handles by falling back
to setSelected(false).

Reviewer note: std::optional<bool> is contextually convertible to bool, so a
bare if (state.selected) still compiles but tests engagement rather than
value, silently marking an explicitly unselected component as selected. There is
a regression test for that specific hazard.

Fixes #46988
Supersedes #47296, which went stale.

Changelog:
[General][Breaking] - AccessibilityState::selected is now std::optional<bool> in C++ props, preserving an unset selected instead of coercing it to false

Reviewed By: javache

Differential Revision: D120049025

[meta-codesync]

@crazeface has exported this pull request. If you are a Meta employee, you can view the originating Diff in D120049025.