Remove reliance on surface to update depth

- Also no longer resets depth on surface change, as depth is independent
  from blur.

This fixes the bug where depth is not accurately reflected.

Bug: 185680609
Bug: 185189176
Test: launch app and swipe home multiple times
      launch app and back gesture to home

Change-Id: I5e2f4ce08b8bf84b7356fbd080ae732875c5e04e