Add navigtaion bar icons to simulate older platforms.

LayoutLib can simulate older versions of the platform to a very limited
extent. This change adds the old navigtaion bar icons back and moves the
latest ones to directories suffixed with "-v21". The code handles this
resolution in a very crude way.

This also removes a bunch of unused code and fixes some style warnings.

Change-Id: Ib6e7bfd8dfb34b2ccd87f660ca979eb5ef14bae7