Fix extreme TextInput slowness on Android (facebook#19645)

Summary:
This reverts 5898817 "Implement letterSpacing on Android >= 5.0".
Testing shows that that commit is the cause of facebook#19126, where in a
controlled TextInput after some text is first added, then deleted,
further interaction with the TextInput becomes extremely slow.

Fixes facebook#19126.

Tried the repro case from facebook#19126 without this change, then with it.
The issue reproduces, then doesn't.
Closes facebook#19645

Differential Revision: D8675230

Pulled By: hramos

fbshipit-source-id: e2c2d352ee781898721b2dff4738572d1a6b7471

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

@@ -119,7 +119,7 @@ private static void buildSpannedFromShadowNode(
                 start, end, new BackgroundColorSpan(textShadowNode.mBackgroundColor)));
       }
       if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
-        if (textShadowNode.mLetterSpacing != Float.NaN) {
+        if (!Float.isNaN(textShadowNode.mLetterSpacing)) {
           ops.add(new SetSpanOperation(
             start,
             end,