Refactoring

- Make internal linkages with unnamed namespace.
- Use `const` instead of `#define` for constants.
- Relocate constructor logic to RegisterWithRegistrar.
- Use helper function `GetValueFromEncodableMap`.
- Use `enum class` instead of `enum`.
- Some naming changes.
- Remove unnecessary logging.
- Initialize private variables inline rather than in constructor.
- Move `HandleResult` from AudioPlayer's memeber to internal.

Author: HakkyuKim