Fixing bad regression in alt-tab layout.

- Removing the notion of drawing the task header thumbnail on preload.
  This would not work because we could not know the stack state until
  showRecents() is actually called.  Instead, we keep a cached thumbnail
  bitmap that we draw into when we start the activity, which is only
  updated when the layout changes.
- Ensuring that with the smaller task views in the focused layout
  overlap and do not show a gap between them (this was introduced when
  the task views were made smaller to show more of the task behind it)
- Ensure that both alt-tab and paging both default to focused state
- Always reset the stack layout to clear the task overrides so that
  we don’t inadvertently get overrides when alt-tabbing

Bug: 28014191
Change-Id: Ibc93597e9c027ce5abd65a8b77c0628864814c9b