Refactor DocsUI to break out a new ManageRootActivity.

- Split all ACTION_MANAGE-related functionality off from
  DocumentsActivity, into a new activity.
- Create new full-screen layouts for said activity.
- Fix some styling issues in {Documents,Files}Activity.

Change-Id: I0384715ad3c7d70a3a3daf510f1a09e8c5732348