Clients have been migrated to the TextInputPlugin.

This functionality has been replaced by the PlatformPlugin. Also move the
contents of //flutter/services/common to more sensible homes.

The platform use cases are handled by the PlatformPlugin. The media service and
the sensor service have no clients. If we want to use them, we should create
plugins for them.

Also, expose a convenience class for processing JSON messages.

We've cut over to the new semantics interface.

Instead, clients should use host messaging.

* 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.