Stop relying on background color attribute to set task transition background

Because the background attribute used to have a different use some apps still set it and it means we could run transitions with unexpected background colors

Test: Existing

Fixes: 200763116
Merged-In: Id42f52760cb9a681c6c33f3c261d7db5e2f71af3
Change-Id: Id42f52760cb9a681c6c33f3c261d7db5e2f71af3