Summary:
This fixes an issue where animations for values near zero could end up formatted
with exponents (e.g. `1.452e-10`), which is not valid for an `rgba` color spec.
This commit arbitrarily rounds it to the nearest thousandth to prevent this type
of formatting while still maintaining high-enough resolution in the alpha channel.
One way this could bubble up to the user is as PropType validation failures:
```
Failed propType: Invalid prop `backgroundColor` supplied to `RCTView`: rgba(0, 0, 0, 9.838983123336224e-7)
```
Closes https://github.com/facebook/react-native/pull/7597
Differential Revision: D3310941
Pulled By: vjeux
fbshipit-source-id: 0c95facaef5b69c021662af9fb6f268d890ecc3e
908041b80b