Add percentage support to react native

Summary:
@public

Adds support for percentage value in react native.

syntax: property: 100 | property | '100%'

supported properties:
padding
margin
width
height
minWidth
minHeight
maxWidth
maxHeight
flexBasis

```
class Playground extends React.Component {
  render() {
    return (
      <View style={{backgroundColor: 'white', padding: 10, paddingTop: 30, height: '100%'}}>
        <Text>
          If you want to quickly test out something,
          open the Playground.js file and start coding.
        </Text>
        <View style={{backgroundColor: 'red', height: 50, width: 50}}/>
        <View style={{backgroundColor: 'blue', height: '50%', width: '50%'}}/>
      </View>
    );
  }
}
```

Reviewed By: astreet

Differential Revision: D4376549

fbshipit-source-id: c41d68a7555396f95d063a7527ee081773ac56dc

java/com/facebook/yoga/YogaValue.java

@@ -19,7 +19,7 @@ public class YogaValue {
   public final float value;
   public final YogaUnit unit;
 
-  YogaValue(float value, YogaUnit unit) {
+  public YogaValue(float value, YogaUnit unit) {
     this.value = value;
     this.unit = unit;
   }

yoga/Yoga.c

@@ -181,8 +181,6 @@ YGCalloc gYGCalloc = &calloc;
 YGRealloc gYGRealloc = &realloc;
 YGFree gYGFree = &free;
 
-static YGValue YGValueUndefined = YG_UNDEFINED_VALUES;
-
 static YGValue YGValueZero = {.value = 0, .unit = YGUnitPixel};
 
 #ifdef ANDROID

yoga/Yoga.h

@@ -43,6 +43,8 @@ typedef struct YGValue {
   YGUnit unit;
 } YGValue;
 
+static const YGValue YGValueUndefined = { YGUndefined, YGUnitUndefined };
+
 typedef struct YGNode *YGNodeRef;
 typedef YGSize (*YGMeasureFunc)(YGNodeRef node,
                                 float width,