Refactor the Pocket category colors into a singular data class

[MozillaNoah]

After the introduction of wallpaper-adjusted colors, PocketCategoriesViewHolder initializes four colors to cover the selection state. For parameter simplification and down-the-road refactoring quality of life, this would benefit from having the colors wrapped in a new data class, such as PocketCategoryColors. This way, a single parameter can be passed around rather than a growing list of colors as the business logic evolves.

The default/fallback colors should be be inlined with the data class and the fallbacks when the passed-in colors are null.

Any nullability handling should be handled with a secondary constructor.

data class PocketCategoryColors(
    val selectedBackgroundColor: Color,
    val unselectedBackgroundColor: Color,
    val selectedTextColor: Color,
    val unselectedTextColor: Color,
)

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