Prevent edge gradient for screenshots in correct orientation

- Due to rounding issues, the width could be a pixel too small, causing
  a gradient on the right edge
- On tall devices, the height ratio of the task thumbnail might be
  larger than the width ratio, causing the screenshot to be too short
  upon swiping up, and thus a gradient on the bottom edge

Bug: 73540853
Change-Id: I1330cec3d2ff8c98eb1b354f0e7f40c6ea49ad7c