Move Adapters to their own classes.

Move section break support into a separate wrapper class.
Fix issue where intermediate directory updates were briefly showing hidden files.
Add a rudimentary test for ModelBackedDocumentsAdapter.

Bug: 26293561, 26383237, 26293561, 26309025
Change-Id: I1fa489b110754d8801091b2009caebe9d2278701
11 files changed