Tests for DocumentsProvider and DocumentsUI.

Developers can launch the OPEN_DOCUMENT, CREATE_DOCUMENT, and
OPEN_DOCUMENT_TREE to let the user pick from DocumentsProvider
instances running on the system.  This test hosts both a provider
and client, and drives various UI interactions to ensure that
these intents match developer expectations.

Bug: 17953507
Change-Id: I40cc5ae7625edc098a43872183376ca1f62d175c