The new alignment option is not enabled by default and may not apply to every books in your library of the Kindle store. To enable left alignment, tap the Aa settings at the top of the screen inside books that support Enhanced Typesetting.
It's great that Amazon added this feature and give the choice to readers to enjoy the best reading experience. Personally, I'll stick to the justified layout. I've grown so accustomed to that layout. Download the update in Google Play Store.