Bring sky_shell.mojo up to feature parity with sky_viewer.mojo

I believe this patch implements all the remaining features from
sky_viewer.mojo.

Skyanalyzer is no longer needed.

Make sky_shell.mojo work on Android

There's no vsync service in MojoShell, so we shouldn't try to connect to it.
Feature request for MojoShell is in https://github.com/domokit/mojo/issues/535.

Fix #371 Replace “chromium” and “sky” with “flutter” in logs and Mac Shell UI

Fix https://github.com/flutter/flutter/issues/371 renaming "sky" and "chromium" to "flutter" in logs and Mac Shell UI

I was supposed to rename this variable before landing the previous patch.

Handle pointer input in sky_shell.mojo

Hold services_provided_to_embedder open

If we let services_provided_to_embedder close, we'll trigger a bug in Dart's
ApplicationConnection framework code. I've filed
https://github.com/domokit/mojo/issues/536 and will fix that separately. This
patch works around the issue for now.

Wire in Mojo services to sky_shell.mojo

After this patch, code running in sky_shell.mojo has access to Mojo services.

Refactor how we wire up platform services

Now the control flow for wiring up platform services starts in the
platform-specific code. Previously we started in shared code, which was
imposing constraints on the startup sequence for all platforms. Now that we
start in platform-specific code, we'll be able to add better support for Mojo
in a future patch.

Remove sky::shell::Engine's dependency on network_service

Wire sky_shell.mojo to draw via MGL

We weren't using the network service for anything.

Inability to find a codesigning identity is not a fatal error

After this patch, we can execute Dart code and draw the result via MGL. We
still need to plumb in services.

Wire sky_shell.mojo up to native_viewport

After this patch, we can execute Dart code an create an X window. There's a
sketch of some graphics code, but it's not wired up yet.

Rev packages again and this time update the mojo_sdk dependency

Make Xcode run `pub get` automatically

Bump sky_engine and sky_services package versions

Roll dart 3