Filter unneeded assets from additional resource directories

This change ensures that assets that are not needed for the
preferred density are stripped from additional directories
specified by the -S flag

For simplicity, the logic for deciding which assets are not
needed is still applied per directory, rather than globally,
which may still result in some unneeded assets being included
in the final output.

Bug: 12604267
Change-Id: Ic1378570c2d9cd1c854c507e784233818c19efca