Adjusts appearance hierarchy for search bar's textfield.

Fixes #2730 

My only concern with this is that this attribute would apply to all UITextFields cointained in UISearchBar throughout the app.  Not sure if there are other search bars in the app, but it can become a pain to figure out what's going on if we decide to add another unrelated one (which should not show this text or use this color).

I'd suggest making this more localized by using, for example:
```objective-c
[[UITextField appearanceWhenContainedIn:[self.view class], [UISearchBar class], nil] setAttributedPlaceholder:attrPlacholderText];
```