[fix] i18n styles

1. Fix auto-flipping of styles

The StyleRegistry didn't account for LTR/RTL when caching the results of
style resolution. The 'writingDirection' style is no longer flipped; no
clear use case for it.

2. Remove experimental '$noI18n' style prop suffix

This feature is essentially unused, and less likely to be used with the
introduction of 'dir=auto' on 'Text'. Removing also marginally improves
render performance.

examples/components/Text/TextExample.js

@@ -271,7 +271,7 @@ const examples = [
         <Text>
           auto (default) - english LTR
         </Text>
-        <Text style={{ writingDirection$noI18n: 'rtl' }}>
+        <Text>
           أحب اللغة العربية auto (default) - arabic RTL
         </Text>
         <Text style={{textAlign: 'left'}}>