Summary:
This PR converts the use of `NativeMethodsMixin` in `CheckBox.android.js`, and converts it to an ES6-style class.
Pull Request resolved: https://github.com/facebook/react-native/pull/21585
Reviewed By: TheSavior
Differential Revision: D12827768
Pulled By: RSNara
fbshipit-source-id: c113c221335e61e015a20bbb6bcff5f9837f9022