Move assets/ to resources/assets/

While it's "semantically" incorrect (these files are not
ResourceLoader resources), putting them in that subdirectory is a lot
less hassle than introducing a new toplevel directory.

Follow-up to 2b4b9a3f. Discussion that resulted in the toplevel
assets/ took place on I6268d663 (now abandoned).

Change-Id: Iedbfd802457fe35803899e3479540177760ec30b