* Add back launch screen view until first frame on iOS
* improvements
* Move callback plumbing from ios surfaces to the gpu rasterizer. Didn’t wire java JNI yet.
* Android JNI
* Fix ios reference count and let android engine manage a view on top with launch screen
* Hook up Android activity and view
* review notes
* review notes
* Move thread switching upstream. Use weak references for callbacks.
* Some clean up
* 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.
