Add boilerplate for compressing in DocumentsUI.

This CL adds strings, constants, and classes for compressing, which
for now do the same as copying.

Along the way it does the same to extracting.

Test: Tested manually.
Bug: 20822019
Change-Id: I9866166c635befae59b3fff13125bd563f71936a
(cherry picked from commit 4f225494)