Round alpha channel when interpolating colors to the nearest thousandth.

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

Libraries/Animated/src/Interpolation.js

@@ -233,7 +233,7 @@ function createInterpolationFromStringOutputRange(
     // 'rgba(${interpolations[0](input)}, ${interpolations[1](input)}, ...'
     return outputRange[0].replace(stringShapeRegex, () => {
       const val = +interpolations[i++](input);
-      const rounded = shouldRound && i < 4 ? Math.round(val) : val;
+      const rounded = shouldRound && i < 4 ? Math.round(val) : Math.round(val * 1000) / 1000;
       return String(rounded);
     });
   };

Libraries/Animated/src/__tests__/Interpolation-test.js

@@ -294,4 +294,14 @@ describe('Interpolation', () => {
       outputRange: ['20deg', '30rad'],
     })).toThrow();
   });
+
+  it('should round the alpha channel of a color to the nearest thousandth', () => {
+    var interpolation = Interpolation.create({
+      inputRange: [0, 1],
+      outputRange: ['rgba(0, 0, 0, 0)', 'rgba(0, 0, 0, 1)'],
+    });
+
+    expect(interpolation(1e-12)).toBe('rgba(0, 0, 0, 0)');
+    expect(interpolation(2 / 3)).toBe('rgba(0, 0, 0, 0.667)');
+  });
 });