Add support for partner customization.

Traditionally Launcher workspace customization is offered through
overlays at build time, but we don't have access to partner-specific
customization at build time.  To solve this, this adds a new
"partner-folder" tag which points at an XML resource provided by
an external package.

The external package XML can't depend on the binary XML attributes
defined by Launcher3, so we switch to using manual string-based
attribute lookups.  Partners can also provide extra wallpapers.

When a folder only results in a single item, promote that item into
the folder location instead of deleting completely.

Bug: 13340779
Change-Id: Ide558288bef4113565f288b700d8245055c0fee9