With the policy being disentangled from application, it becomes testable.
This prepares for moving the entire visibility mechanism to the new class and taking away more pieces of ServerContextService.
In addition, this is a good warmup before trying to implement sizing policy.
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.
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.
