RTL positioning tweaks

* partially undo the RTL fix from mingle #512 which looks bad,
  and use four-value syyntax for background-position instead.
* use noflip on :before rule as :before itself is not flipped
* handle better the uncertainty resulting from image name
  usually not being RTL

Change-Id: If4aa7240e9bee5c0a06f1785edb9c72964dfc60f
Merge "RTL positioning tweaks"