As stated in Kotlin Coding Conventions. Closes #42212

* Remove use_modular_headers from Podfiles using libraries

* Remove @imports

* Test the Android embedding v2

* Update integration tests

* Split into shards

* Add smoke test for the new Android embedding

* Update AndroidManifest.xml in app template

* Update test README.md

* Remove widget_test.dart

* Update pubspec.yaml

* Force GeneratedPluginRegistrant.java

* Generate projects using the new Android embedding

* Add comment about usesNewEmbedding:true

* Feedback

* Rework way to detect new embedding in new apps

Adds macOS support for `flutter create`:
- Currently it is behind a hidden flag.
- Adds a TargetPlatform workaround to lib/main.dart in the standard app template when enabled.
- Supports `app` and `plugin`; `module` support doesn't yet exist for macOS in general.

This will eliminate the need to use FDE's examples as templates on macOS. The templates are based on the current state of FDE's examples, with templating support added (and with adoption of the new application delegate in the app, which hadn't been done yet in FDE, eliminating some boilerplate from the template).

Fixes #30703 

Rather than have a monolithic .gitignore covering all platforms in the
app template, have each platform folder contain its own rules.

Fixes #40793

* Fix plugin template

* Add test of plugin template working

Change the target macOS version from 10.13 to 10.11, per decision on
target platform.

Part of #33200

If xcode_backend.sh script fails or substitute variables are missing, fail the host Xcode build (#37449)