Type-to-focus improvements.

- Fix handling of tab and backspace keys while in type-to-search mode.
- Fix search term highlighting on directory items in grid view.
- Switch to a time-based search model.  Typing starts a search.  The
  current search expires if no keys are pressed for 500 ms.
- Make type-to-focus case-insensitive.

BUG=27336167
BUG=27324974

Change-Id: I92633222c9a04af24729501c48885eabebf00696