Create a universal constant for minimum interactive size

[justinmc]

Lots of places in the codebase have a hardcoded value of 48x48 pixels (material) or 44x44 pixels (cupertino) in order to size hit targets in a way that's easily interactive. We should create a single constant for these and reuse it everywhere.

See https://github.com/flutter/flutter/pull/35829/files#r305981652 for discussion and locations.

[github-actions]

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