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This change makes all downloads through the public API visible by
default.  It removes the API that had allowed applications to control
notifications while the download runs.  This has been replaced with a
hidden API, since such behavior is needed by SystemUpdater and Market
(for self-updates).  Additionally, the behavior is now protected by a
new permission.

I'm making this permission signatureOrSystem, and changing the
non-purgeable permission to the same (it should've been that, I just
didn't know).  I'm also adding string descriptions to appease the
translation folks.

Change-Id: I192e8b19ff9b0e425257cef0db081c3d75996ea5