Rechristen Downloads > Files.

Enable productivity mode by default.
  - New window support.
  - Documents root is shown and is home dir.
Use the Files icon, instead of downloads.
Cleanup Shared.foo settings logic a bit.
Eliminate use of ENABLE_PRODUCTIVITY_MODE (but don't remove just yet).

This will let folks like Ronald Ho test from master.

Change-Id: I038828ac398ec0625db033aa576b6951ea450736