3579 [image_picker] NNBD stable (cla: yes, nnbd, p: image_picker, waiting for tree to go green)

3580 [image_picker] use nnbd version of deps to resolve ci failure (cla: yes, nnbd, p: image_picker)

3582 [path_provider] Update to stable NNBD (cla: yes, nnbd, p: path_provider)

3583 [url_launcher] Fix SDK copypasta in platform interface (cla: yes, nnbd, p: url_launcher)

3584 [url_launcher] Update platforms to NNBD stable (cla: yes, nnbd, p: url_launcher, platform-linux, platform-macos, platform-web, platform-windows)