Move payment icons from luminance-based selection to ThemeMode behind a feature flag

[toluo-stripe]

Summary

Move payment icon selection away from component luminance and toward the resolved PaymentSheet.Appearance.themeMode. With FeatureFlags.disableLuminanceBasedThemes enabled, the effective theme determines light/dark artwork, icon URLs, tint, and the inline Affirm logo, even when custom component colors have different brightness.

The flag defaults off.

Icon behavior

With Disable Luminance Check enabled, component brightness does not affect icon selection:

ThemeMode	Current system theme	Effective theme / color palette	Artwork / URL variant	Resource tint, when required
Automatic	Light	Light / colorsLight	Light / day	Black
Automatic	Dark	Dark / colorsDark	Dark / night	White
AlwaysLight	Light	Light / colorsLight	Light / day	Black
AlwaysLight	Dark	Light / colorsLight	Light / day	Black
AlwaysDark	Light	Dark / colorsDark	Dark / night	White
AlwaysDark	Dark	Dark / colorsDark	Dark / night	White

Motivation

A dark theme with a bright component background currently selects light-theme artwork and black icon tint. Enabling the flag selects dark-theme artwork and white tint for the same background, allowing icon behavior to be tested independently of custom component colors. Forced AlwaysLight and AlwaysDark modes take precedence over the system theme.

The PaymentOption.icon() fallback also previously consulted the system theme and mutable global colors rather than the supplied appearance. It now honors that appearance consistently.

Testing

• Added Affirm screenshots: 16 snapshots covering both flag states, light/dark themes, default/large fonts, and default/opposing component colors. Affirm and CardPill verification passed (7 cases), and :payments-ui-core:detekt passed.
• Added tests: 21 flag-on snapshots across tab, row, and embedded content, covering both font sizes for tabs and rows. Existing flag-off baselines are preserved.
• Modified tests: cover both flag states with forced themes and opposing component brightness, both icon() and iconPainter, resource/URL selection, and the appearance-based fallback.
• Manually verified: inspected representative generated snapshots for the expected icon tint changes.

Screenshots

Dark system theme with the bright yellow CrazyAppearance component:

Before / flag off	After / flag on

Committed and created by Codex.