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I need to make COLUMN_URI readable by apps, since the public API
exposes that field.  In order to avoid any possible security issues, I
got rid of the feature that potentially allowed apps to view downloads
from other UIDs.  No one was using that feature and the public API
exposes no such feature (yet).

While at it, I cleaned up some related code in update() and delete().

Change-Id: I5384115d2a865255d009fbe37449488fd2269389