Fix extreme TextInput slowness on Android

Because NaN is special, the `!=` version of this condition will
always be true -- even if `mLetterSpacing` is also `Float.NaN`, which
is its default value.  It should instead be `!Float.isNaN(...)`.

The effect of the broken condition is that every text shadow node will
create a `CustomLetterSpacingSpan` around its contents, even when the
`letterSpacing` prop was never set.

Empirically, that in turn causes facebook#19126: in a controlled TextInput
after some text is first added, then deleted, further interaction
with the TextInput becomes extremely slow.  Perhaps we're somehow
ending up with large numbers of these shadow nodes (that sounds like
a bug in itself, but I guess it's normally low-impact); then this
code would have caused them each to generate more work to do.  In
any case, fixing this condition causes that issue to disappear.

The affected logic was introduced between v0.54 and v0.55, in facebook#17398
aka 5898817 "Implement letterSpacing on Android >= 5.0".

Fixes facebook#19126.

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,