[flutter_tools] remove globals from FlutterValidator, add documentation and move tests to new file (#67234)

Remove globals from the flutter validator class, and refactor the tests into a separate file. Applies some other cleanup like adding doc comments, and making the doctor validator work like it is documented to work - removing the gen_snapshot check if the artifact is not downloaded instead of downloading all android artifacts.

#47161

The flutter tool is currently unable to detect missing permissions in gradle/gradle.bat that would cause a gradle build to fail via process exception. Rather than crashing and exiting, we can display the exception as an error message and tool exit.

While linux/macOS are able to add the +x bit, this is not possible on windows with our current file system/OS API. These crashes represent a substantial amount of crash reporting, but are otherwise not actionable on our end.

Make it possible for all FlutterCommands to be global free, by moving instantiation to inside the Zone context. Additionally, provide VerboseLogger and NotifyLogger (daemon) at the top level and remove from command-specific overrides.

This allows removing a work around where web devices needed to look up directly from the context in non-test code.

Technically the output preferences are still zone injected, but these will be moved soon as they were not being used correctly by the top level command (the injection comes after ArgParser reads the overflow values, causing numerous wrap issues)

The `runZoned` method deprecates the `onError` parameter.
Invocations using that parameter must use `runZonedGuarded` instead.

This prepares Flutter for the deprecation. The same places need to be migrated to using
`runZonedGuarded` when the SDK change has been ported to Flutter.
Then the deprecated member will be removed in a follow-up CL when the most important
packages have been migrated.

This reverts commit 0a38f29c.

* Update project.pbxproj files to say Flutter rather than Chromium

Also, the templates now have an empty organization so that we don't cause people to give their apps a Flutter copyright.

* Update the copyright notice checker to require a standard notice on all files

* Update copyrights on Dart files. (This was a mechanical commit.)

* Fix weird license headers on Dart files that deviate from our conventions; relicense Shrine.

Some were already marked "The Flutter Authors", not clear why. Their
dates have been normalized. Some were missing the blank line after the
license. Some were randomly different in trivial ways for no apparent
reason (e.g. missing the trailing period).

* Clean up the copyrights in non-Dart files. (Manual edits.)

Also, make sure templates don't have copyrights.

* Fix some more ORGANIZATIONNAMEs

* On flutter tool crash suggest GitHub template

* Shorten GitHub URLs

* Whitespace

* Wording tweak from @InMatrix

* Review edits

* implicit-casts:false on flutter_tools

* use castStringKeyedMap

* address review comments

* address review comments

* fix issues after rebase

This reverts commit 3597bae6.

...because otherwise, processes that think they're manipulating your
filesystem will be doing crazy things the test is ignoring, leading to
(at best) failures and (at worst) flakes or disk corruption.