New aapt feature to do smarter filtering of configurations.

This adds a --preferred-configurations flag that specifies the
specific configurations you would like to have.

It is smarter than "-c" because it will avoid stripping a
configuration if that would result in there being no value
for the resource.

It is dumber than "-c" because it can't process as many kinds
of resources.  It is really only intended for bitmaps and use
with density configs.

This required re-arranging AaptAssets to group files together
by config again, like they used to be.  I think this hasn't
broken anything.  Hopefully.

Change-Id: I4e9d12ff6e6dbd1abb8fd4cb1814c6674b19d0e5