[url_launcher_web] Remove package:platform_detect

[ditman]

Description

This change removes the dependency that url_launcher_web has in package:platform_detect to achieve a null-safe gallery app.

It also migrates the unit tests to run with flutter drive, instead of flutter test with --platform.

Related Issues

• Fixes [url_launcher_web] Remove dependency in platform_detect package... flutter#66385

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Breaking Change

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[blasten]

LGTM for the dependency drop