New command line tool: "settings"

Because the usual content provider interface doesn't allow specification of
the target user under which to perform settings reads/writes, this CL introduces
a new command line app specifically for that.

Usage:

   settings [--user num] get namespace key
   settings [--user num] put namespace key value

If a --user argument is not given, the owner user is targetted.

Bug 7299066

Change-Id: I73108bb76b04fad133cc4e0f218d64490de549f1