Only set task surface background in freeform

Only set task surface background color when task windowing mode is set
to freeform.
Unset task surface background color when task windowing mode is
fullscreen.

Since setting the task surface background color is needed for fluid
resizing, only set it when it is enabled.

Bug: 303871142
Test: atest WindowDecorationTests
Change-Id: I361207413d748dbe99afcd3da286da79ff4856cd