3891 Begin migrating tools to NNBD (cla: yes)

3892 [video_player][camera] fix video_player camera tests (cla: yes, p: camera, p: video_player)

3893 [url_launcher] Migrate pushRouteNameToFramework to ChannelBuffers (cla: yes, waiting for tree to go green, p: url_launcher)

3894 Bump min Android SDK to the version required at runtime (cla: yes, waiting for tree to go green, p: webview_flutter, p: google_maps_flutter, platform-android)

3896 [in_app_purchase] Add reference to the codelab in the README (cla: yes, p: in_app_purchase)

3897 [in_app_purchase] Expose the NSLocale object (cla: yes, p: in_app_purchase, platform-ios)