Add functional tests for FilesActivity.

Adds a bare bones test provider with dummy data.
This is basically just the bones of a test at this point.
Has just one simple test...switch list/grid modes.
Will add more tests (like manipulating the dummy data) in future CLs.

Bug: 24988170
Change-Id: Icc25718a416bc804874835c74a6d7862a52c9861