Initialize getConstantsForViewManagers eagerly

Summary: During the JS run, the view managers used on a React Native screen eventually call the native methods for `UIManagerModule.getConstantsForViewMangers(viewManagerName)`. This blocks the JS thread. This diff tries to cache the values of those calls and return them when JS needs it, ensuring that JS is not blocked as much.

Reviewed By: achen1

Differential Revision: D9985817

fbshipit-source-id: 36feabc8a386956f8a6474f6e7978285d31f24dd

Author: axe-fb