Fix default shadow radius in TextAttributeProps (facebook#36621)

Summary:
Pull Request resolved: facebook#36621

After D44302691 enabled textShadow, there was a subtle 1px shadow on any new text which I did't spot, but screenshot tests did (after commit which is non-ideal, but there is more work to make these land blocking).

This is because unlike `ReactBaseTextShadowNode` in paper which defaults to a radius of zero (no shadow), `TextAttributes` in Fabric defaults to a radius of 1px. Just previously never displayed.

Without shadow:
https://pxl.cl/2z2wX

With shadow:
https://pxl.cl/2z2x0

This changes the default to zero, which will cause us to skip adding the span, and matches previous behavior in Paper.

I double-checked the other props are defaulted the same way between `BaseTextShadowNode` (Paper) and `TextAttributes` (Fabric).

Changelog:
[Android][Fixed] - Fix default shadow radius in TextAttributeProps

Reviewed By: javache

Differential Revision: D44364446

fbshipit-source-id: d207367608291048001403d292f881c0842113f9

Author: NickGerleman

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

@@ -96,7 +96,7 @@ public class TextAttributeProps {
 
   protected float mTextShadowOffsetDx = 0;
   protected float mTextShadowOffsetDy = 0;
-  protected float mTextShadowRadius = 1;
+  protected float mTextShadowRadius = 0;
   protected int mTextShadowColor = DEFAULT_TEXT_SHADOW_COLOR;
 
   protected boolean mIsUnderlineTextDecorationSet = false;