Changed assigned once let(s) to const and simplified shouldAccept/InterceptHitEvent functions
Changed assigned once let(s) to const and simplified shouldAccept/InterceptHitEvent functions

Summary:
## Motivation (required)
I'm reading OVRUI code since it was merged yesterday. I've come to terms that I probably will never understand it (:smile:) but I've noticed that there are some `let` statements that could easily be `const` since they are never reassigned again. If that's intentional - please feel free to close this PR.

I've also simplified two if statements in `shouldAcceptHitEvent` and `shouldInterceptHitEvent` functions :)

## Test Plan (required)
All tests pass when running `npm run test`
Closes https://github.com/facebook/react-vr/pull/157

Reviewed By: andrewimm

Differential Revision: D4949878

Pulled By: andrewimm

fbshipit-source-id: fa7cab2

ichengde/react-vr,ichengde/react-vr