Scale text line height according to system font size

Summary:
## Changelog:
[Android] [Added] - Added getter for line height in CustomLineHeightSpan

Reviewed By: javache

Differential Revision: D41324647

fbshipit-source-id: 4c2b18fb6566f5f9f25e26acb2f7f157f4ab75f8

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

@@ -21,6 +21,10 @@ public class CustomLineHeightSpan implements LineHeightSpan, ReactSpan {
     this.mHeight = (int) Math.ceil(height);
   }
 
+  public int getLineHeight() {
+    return mHeight;
+  }
+
   @Override
   public void chooseHeight(
       CharSequence text, int start, int end, int spanstartv, int v, Paint.FontMetricsInt fm) {