Implement enter key support for DocumentsUI.

- Add a click handler that basically handles all cases (including the
  enter key) of Items being clicked.
- Open the focused file in non-select mode, toggle selection if in
  selection mode.
- Remove now-redundant onSingleTapUp handlers.
- Redo focus indicator, since it's now possible to toggle selection on
  the focused file.
- Don't dismiss selection on arrow-key navigation.

BUG=24705453
BUG=25114155

Change-Id: Iba74188514ecf2bc6b56573087e3c3ccc83f85d2