Introduce a non-SDK class to hold the implementation details

This is an exact clone of the "public" class, so that applications
that access the raw content provider directly (i.e. Browser) can
continue doing so, at the expense of not being buildable with the
SDK.

The Download Provider itself will also use those constants.

Bug: 2245521

Change-Id: I15f8ca8a3b69719cfbf403fc2607aac0d3c2b79a