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

3644 [path_provider] Migrate all integration tests to NNBD (cla: yes, p: path_provider, platform-linux, platform-macos, platform-windows)

3650 Update plugin_platform_interface min version (cla: yes, p: battery, p: camera, p: connectivity, p: device_info, p: file_selector, p: google_maps_flutter, p: image_picker, p: path_provider, p: url_launcher, p: wifi_info_flutter)

3672 [path_provider] Make a 0.0.5 for Dart 2.12 compat (cla: yes, p: path_provider, platform-windows)