Combining the Thumbnail bitmap with the background to fix background spilling in the front.

What's happening is that we first draw a background,
on top of it, we then draw the Task and because we
have a curvature the corners have pixels with transparency,
and those pixels get combined with the background pixels.
Most tasks have a transparent background or background
of the same color as the task so we don't see this
but for example, Telegram has a white background and
you can see this in the corners.

Fixed by creating a bitmap, drawing the background on it
and then drawing the thumbnail on top of it then using
that bitmap to draw it on the canvas.

Test: Put Telegram in recent Task (shouldn't be the first one) and you won't see a white border. Also when a task was in multitask the backgroudn should be draw.
Fix: 146521490
Change-Id: Id02a64ef472eb07900b0c7c5522d931d5b08f94e