Remove the task background surface

The decoration container surface was used to capture input events
outside the task bounds, which required the task surface to be larger
than the actual task size and thus a different background surface was
needed to draw the backrground color on just the task size.
Since outsets are no longer needed per
Ib9764cf17f23ef9b5688ead7ae93dabbc1155049, the task surface size now
matches the real task size and the background color can be drawn on
that same surface, making the background surface obsolete.

Bug: 270202228
Test: atest WindowDecorationTests
Test: open freeform window, verify background is visible

Change-Id: I063cda38ba1aebd1473d1b9522f4a54e2a5c095c