Removes these two discontinued plugins from `dev/integration_tests/flutter_gallery`
[`device_info`](https://pub.dev/packages/device_info):
Apparently the video playback doesn't work on iOS simulators (I wasn't able to verify this, as I don't have an iOS simulator installed). I removed the guard against running on those simulators, and replaced with a note in the README.
[`connectivity`](https://pub.dev/packages/connectivity):
This plugin was used to play the bee video from the network. I changed the demo so that the bee video is instead also played from an asset (like its friend the butterfly), and then removed the use of the plugin.
Unblocks the re-land of https://github.com/flutter/engine/pull/53462 (itself a reland ð), because of https://github.com/flutter/flutter/pull/150465#issuecomment-2181403712.
Adds Navigator.maybeOf to replace calling Navigator.of(context, nullOk: true), and removes the nullOk parameter. Also changes Navigator.of to return a non-nullable value, and removes many (120!) instances of the ! operator, reducing the possible places where a null dereference could occur.
Move Flutter Gallery to dev/integration_tests/ as it is an older copy used only for testing. The current version of the Flutter Gallery now lives in https://github.com/flutter/gallery.
