Prune unneeded density resources from APK

When a preferred density is specified, prune all but the
closest matching density when the exact density resource does not
exist.

If a preferred density resource does not exist and a higher density
resource exists, prune all but the resource with the next highest
density relative to the preferred density.

If a preferred density resource does not exist and there is no
higher density resource, prune all but the resource with the next
lowest density relative to the preferred density.

bug:11224465
Change-Id: I99b3950fe3d8116a5625968ed383442315ae8526