Fix keyboard focus for opening folder.

Previous patch Ia4ccc59ca27e560e470122d1b2c6c1eccb4caf9a was
reverted due to a weird behavior around folder name editing
with virtual keyboard.
Also, requestFocus() on doneEditingFolderName() did nothing
because the folder itself didn't get the focus at that point.

This CL addresses these issues:
- no change on the descendant focusability. Folder itself is
  focusable before its descendants.
- customize Folder.focusSearch(), so once the folder is focused,
  further focus cycle moves among its descendants.
- Folder requests its focus when it appears. Without this,
  the first arrow-key after opening a folder by touch moves
  the focus to the folder itself, which looks like nothing happened.

Bug: 22562679
Change-Id: I04375bd4cb69d463b9a95bebafa9c60142e95daa