After this patch, platform messages now take the same path through the system
that semantics data does (on Android). Support on iOS will be in another patch.
We now use JNI rather than Mojo to transport pointer data from Java to Dart.
Also, fill in a few more of the pointer data fields from information in Java.
* 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.
