docs: change download URLs for design assets.

These new URLs are dummies that are handled by a redirect rule for
all /downloads/... paths, that points to http://commondatastorage.googleapis.com/androiddevelopers/
Using this redirect allows us to change the storage location of the download assets in the future.

Change-Id: I0dc6ecc1a6898f1256ba4c9ff90b619366e47629