Rework selection handling for items in the DirectoryFragment.

- Remove the gesture detector from the MultiSelectManager, and
  consolidate gesture detection and event dispatch logic in
  DirectoryFragment.GestureListener.

- Route single-tap events through the DocumentHolder, so that it can
  apply view-specific logic, like making a tap on the item's icon
  select rather than activate.

- Consolidate event handling logic in the ItemEventListener.

- Add new unit tests for DocumentHandler.

BUG=24326546

Change-Id: Id15cdd11b13e4c063c1baff95aa8ee09c190d6c3