[flutter_adaptive_scaffold] use MediaQuery.sizeOf instead of MediaQuery.of to prevent unnecessary rebuilds

[waleedf112]

the isActive method uses MediaQuery.of(context).size.width, which triggers widget rebuilds whenever a property of MediaQuery changes, even when the size.width doesn't change.

instead, we should be using MediaQuery.sizeOf(context).width.

fixes: flutter/flutter#146801

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