Use black only for desktop tasks background

Use black instead of taskDescription backgroundColor for temporary
workaround so exit from DW for transparent activites with light theme
don't have white background. This should be re-added when
WindowDecoration2 refactor is completed.

Flag: EXEMPT bug fix
Bug: 432347028
Test: jank CUJ test in b/432976351
Change-Id: I676e42226de7a3f67eee288ca610ebb3d299cbc9