[webview_flutter] Replace deprecated Color.value

[stuartmorgan-g]

Replaces the deprecated Color.value with the equivalent Color.toARGB(). For iOS, where this is not the ideal long-term solution, adds a TODO for a better replacement.

Part of flutter/flutter#159739

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Footnotes

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[gemini-code-assist]

Code Review

This pull request correctly replaces the deprecated Color.value with Color.toARGB32() across the Android and WKWebView implementations. The changes in the production code and tests are appropriate. However, I found a critical issue in webview_surface_android.dart where the logic to detect semi-transparent colors for enabling hybrid composition is incorrect, which could lead to rendering issues.

[gemini-code-assist]

The condition backgroundColor.a < 1.0 for enabling hybrid composition seems incorrect. The Color.a property is an integer ranging from 0 to 255, representing the alpha channel. This condition will only be true for a fully transparent color (a == 0), but not for other semi-transparent colors.

The original condition backgroundColor.opacity < 1.0 was correct. If you want to use the integer alpha value, the condition should be backgroundColor.alpha < 255.

Suggested change

	backgroundColor != null && backgroundColor.a < 1.0,
	backgroundColor != null && backgroundColor.alpha < 255,

[stuartmorgan-g]

The Color.a property is an integer ranging from 0 to 255

This is a lie, it's a double from 0-1.

[bparrishMines]

LGTM