Fix negative letterspacing in Fabric android

Summary:
This diff adds support for negative letterspacing values in Fabric android
Changelog: add support fot negative letterspacing values in Fabric android

Reviewed By: JoshuaGross

Differential Revision: D18054648

fbshipit-source-id: de1b906e6b8c0b021ffdc2e4bdad243075230667

ReactAndroid/src/main/java/com/facebook/react/views/text/TextAttributeProps.java

@@ -35,7 +35,6 @@ public class TextAttributeProps {
   private static final int DEFAULT_TEXT_SHADOW_COLOR = 0x55000000;
 
   protected float mLineHeight = Float.NaN;
-  protected float mLetterSpacing = Float.NaN;
   protected boolean mIsColorSet = false;
   protected boolean mAllowFontScaling = true;
   protected int mColor;
@@ -197,10 +196,17 @@ public class TextAttributeProps {
 
   public void setLetterSpacing(float letterSpacing) {
     mLetterSpacingInput = letterSpacing;
-    mLetterSpacing =
+  }
+
+  public float getLetterSpacing() {
+    float letterSpacingPixels =
         mAllowFontScaling
             ? PixelUtil.toPixelFromSP(mLetterSpacingInput)
             : PixelUtil.toPixelFromDIP(mLetterSpacingInput);
+
+    // `letterSpacingPixels` and `mFontSize` are both in pixels,
+    // yielding an accurate em value.
+    return letterSpacingPixels / mFontSize;
   }
 
   public void setAllowFontScaling(boolean allowFontScaling) {

ReactAndroid/src/main/java/com/facebook/react/views/text/TextLayoutManager.java

@@ -74,10 +74,10 @@ public class TextLayoutManager {
                   start, end, new ReactBackgroundColorSpan(textAttributes.mBackgroundColor)));
         }
         if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
-          if (!Float.isNaN(textAttributes.mLetterSpacing)) {
+          if (!Float.isNaN(textAttributes.getLetterSpacing())) {
             ops.add(
                 new SetSpanOperation(
-                    start, end, new CustomLetterSpacingSpan(textAttributes.mLetterSpacing)));
+                    start, end, new CustomLetterSpacingSpan(textAttributes.getLetterSpacing())));
           }
         }
         ops.add(