Layout management was pointlessly bound with the EekboardContextService with inheritance. Splitting it out will make it easier to further break apart layout state management, settings, and input method in the future.
The structure is defined in Rust, with the intention of evaluating using Rust in this area. It's specifically not defined as repr(C), in order to encourage that. Without the repr, it was unsafe to have its members exposed in C.
This adds `bitflags.rs` from the bitflags crate.
Due to not wanting to introduce Cargo as the dependency manager yet, it's slightly modified to compile as a naked module.
