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
e6b680364d