Convert `AndroidSemanticsAction` to enum.

This reverts commit 0df0e2ea.

This reverts commit 5fd259be.

This reverts commit 1002a768.

This reverts commit 2be0d57f.

This reverts commit 9846fa51.

This reverts commit 0100285e.

This reverts commit 0d1a8a77.

This reverts commit 0cbe5975.

This reverts commit 8de07d55.

[a11y] Make sure RenderFractionalTranslation updates its semantics after the translation field is set (#48985)

* 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

Added tests for the new Android heading semantic flag to android_semantics_testing.