fix(appbar): make back icon respect the icon from Provider settings

[giaBaoJS]

Motivation

Icon is the only component that reads the icon entry from SettingsContext:

react-native-paper/src/components/Icon.tsx

Lines 145 to 158 in df3cdfd

	} else if (typeof s === 'string') {
	return (
	<SettingsConsumer>
	{({ icon }) => {
	return icon?.({
	name: s,
	color: iconColor,
	size,
	direction,
	testID,
	});
	}}
	</SettingsConsumer>
	);

A number of components import MaterialCommunityIcon directly instead, which bypasses that consumer. Because src/core/settings.tsx#L18 also uses MaterialCommunityIcon as the default context value, those call sites look correct until an app actually passes settings={{ icon }} to PaperProvider — at which point the override is silently ignored.

Appbar.BackAction is one of the two components named in #4760.

Scope

Deliberately narrow: this PR only changes Appbar/AppbarBackIcon.tsx.

The same bypass exists at these call sites, which I left alone because they all sit under active/competing PRs and are better done separately (happy to follow up):

• Chip/Chip.tsx (2)
• Snackbar.tsx (1)
• List/ListAccordion.tsx (1)
• DataTable/DataTablePagination.tsx (4)
• DataTable/DataTableTitle.tsx (1)
• RadioButton/RadioButtonIOS.tsx (1)
• Searchbar.tsx (2)

Also out of scope on purpose: the Platform.OS === 'ios' branch of AppbarBackIcon renders the bundled back-chevron.png asset rather than an icon-font glyph. Routing that through settings.icon would replace the iOS chevron with an arrow-left glyph for every app that does not customise icons, which is a visual change I did not want to smuggle into a bug fix. Let me know if you'd like it changed and I'll do it here or in a follow-up.

Note on RTL

The object form { source: 'arrow-left', direction } is used rather than the bare string "arrow-left". Icon computes direction as null for a bare string source, which drops the scaleX: -1 mirroring that the previous direction={direction} prop provided — the back arrow is direction-sensitive, so that would have been a regression. There is a dedicated test for both RTL and LTR to keep it that way.

Test plan

Added three cases to src/components/__tests__/Appbar/Appbar.test.tsx (Platform.OS is forced to android because the jest preset defaults to ios, where this branch is not reachable):

• the icon from PaperProvider settings={{ icon }} is rendered
• it stays mirrored in RTL
• it stays unmirrored in LTR

Before the fix all three fail (the default arrow-left glyph renders instead of the custom one). With the bare-string form, the RTL case fails on its own.

yarn test: 55 suites, 738 passed / 1 skipped, 169 snapshots — no snapshot churn. yarn lint and yarn typecheck clean.

Fixes part of #4760.