* [iOS] Ensure FlutterMain is called before interacting with the engine in any way.

* Licenses

We no longer use mojom to transport messages. We still use the Mojo EDK
to spin the event loop, however.

* Namespaces have been updated to reflect the move from //flutter/sky/shell to //flutter/shell.
* shell/BUILD.gn file has been split into smaller GN files for each subcomponent of the shell (common, GPU, diagnostic, testing).
* GN dependencies have been rewritten to stop exposing common shell dependencies as public. Duplicates have also been removed.
* GPU subcomponent has been updated make it more suitable for Vulkan integration.
* The GLFW backend has been resurrected.

After this patch, main_ios.mm now consumes Flutter.framework via public APIs.
This patch prepares us to move Flutter on iOS over to consuming
Flutter.framework as a binary artifact.

All the headers for Flutter.framework are now in framework/Headers and all the
source files for Flutter.framework are now in framework/Source. Previously it
was unclear that FlutterAppDelegate and main_ios.mm weren't part of
Flutter.framework. (They still build as part of the framework, but that's
something I'll fix in a later patch.)

SkyShell now supports --packages instead of --package-root.

Also fixes an issue where a error would get logged even though loading from the precompiled application bundle was successful.

If called via `bundleWithIdentifier:` the bundle may not have been loaded. This makes it explicit.

Also cleans up the messages when errors occur in both modes. I have also made the API a lot simpler and moved more of the extraneous stuff out of public. Only FlutterViewController and the Dart project configuration are publically available.