Don’t scale task thumbnails.

- Changing task view thumbnail layout.  In portrait, scale the thumbnail
  to width for portrait screenshots, and apply the same scale to 
  landscape screenshots.  In landscape, scale screenshots up to 1:1, and
  tweak the app transition to clip the sides instead of scaling.
  In both orientations, fill with the background color in the remaining
  space.
- Moving some resources related to the title bar to be calculated 
  programmatically so that we can have different header bar sizes which 
  completely overlap the action bar in the screenshot in each 
  orientation.
- Constraining the task stack width in landscape to portrait

Bug: 27504677
Change-Id: Ic9b6fdde6dd728d9f2d20a8b89c05b3a350edfbf