Fix spannable flags in RCTVirtualText

Summary:
The spannable flags for RCTVirtualText were always being set to
INCLUSIVE_EXCLUSIVE - this is different than the behavior that is found in
ReactTextShadowNode, which sets EXCLUSIVE_INCLUSIVE as the default flag
unless the text is at the beginning.

This is needed to fix various problems with TextInput, including the handling
of empty spans (see also D2962394 which depends on this patch), and making the
behavior consistent when styled children of a TextInput are changed.

Differential Revision: D2962643

ReactAndroid/src/main/java/com/facebook/react/flat/RCTVirtualText.java

@@ -50,11 +50,17 @@ import com.facebook.react.uimanager.ViewProps;
   protected void performApplySpans(SpannableStringBuilder builder, int begin, int end) {
     mFontStylingSpan.freeze();
 
+    // All spans will automatically extend to the right of the text, but not the left - except
+    // for spans that start at the beginning of the text.
+    final int flag = begin == 0 ?
+        Spannable.SPAN_INCLUSIVE_INCLUSIVE :
+        Spannable.SPAN_EXCLUSIVE_INCLUSIVE;
+
     builder.setSpan(
         mFontStylingSpan,
         begin,
         end,
-        Spannable.SPAN_INCLUSIVE_EXCLUSIVE);
+        flag);
 
     if (mShadowStyleSpan.getColor() != 0 && mShadowStyleSpan.getRadius() != 0) {
       mShadowStyleSpan.freeze();
@@ -63,7 +69,7 @@ import com.facebook.react.uimanager.ViewProps;
           mShadowStyleSpan,
           begin,
           end,
-          Spannable.SPAN_INCLUSIVE_EXCLUSIVE);
+          flag);
     }
 
     for (int i = 0, childCount = getChildCount(); i < childCount; ++i) {