Fix TextInput text color not applying

Summary:
Setting the color for a TextInput with nodes was broken. The text color was
not being applied due to an optimization that prevented setting spans if
begin and end were the same (which is the case for an empty TextInput).
This patch depends on D2962643.

Differential Revision: D2962394

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

@@ -52,12 +52,24 @@ import com.facebook.react.uimanager.ReactShadowNode;
     mTextEnd = builder.length();
   }
 
+  /**
+   * Whether or not to allow empty spans to be set
+   * This is used to bypass an optimization in {@code applySpans} that skips applying spans if
+   * there is no text (since, for TextInput, for example, we want to apply the span even if there
+   * is no text so that newly typed text gets styled properly).
+   *
+   * @return a boolean representing whether or not we should allow empty spans
+   */
+  /* package */ boolean shouldAllowEmptySpans() {
+    return false;
+  }
+
   /**
    * Recursively visits FlatTextShadowNode and its children,
    * applying spans to SpannableStringBuilder.
    */
   /* package */ final void applySpans(SpannableStringBuilder builder) {
-    if (mTextBegin != mTextEnd) {
+    if (mTextBegin != mTextEnd || shouldAllowEmptySpans()) {
       performApplySpans(builder, mTextBegin, mTextEnd);
     }
   }

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

@@ -140,6 +140,11 @@ public class RCTTextInput extends RCTVirtualText implements AndroidView, CSSNode
     mPaddingChanged = false;
   }
 
+  @Override
+  boolean shouldAllowEmptySpans() {
+    return true;
+  }
+
   /**
    * Returns a new CharSequence that includes all the text and styling information to create Layout.
    */