More device admin work: description, policy control.

There is now a description attribute associated with all components,
that can supply user-visible information about what the component does.
We use this to show such information about device admins, and wallpapers
are also updated to be able to show this in addition to the existing
description in their meta-data.

This also defines security control for admins, requiring that they
declare the policies they will touch, and enforcing that they do
so to be able to use various APIs.