Fix issue with resizing background and incorrectly outline.

- Revert to simple color drawable to ensure that we still see a visible
  outline in Overview while it is resized in split screen. We can't use
  the gradient drawable in this case because both the window background
  and the background in the backdrop frame renderer are visible, which
  would look weird.
- Also use the screen dimensions for both drawables to ensure that we
  get the right outline bounds for the DecorView when the window bg
  bounds change.

Bug: 64645364
Test: Enter split with overview, dismiss docked app and ensure that it
      draws under the nav bar.

Change-Id: I07c885f1148163ff59876f3bd2448655fe5928a1