VirtualDisplayTaskEmbedder: Post background color change callback

Otherwise we will end up calling SurfaceView#setResizeBackgroundColor
from a Binder thread and potentially cause a crash. This mirrors the
behavior of TaskOrganizerTaskEmbedder.

Bug: 157605189
Test: Existing tests pass
Change-Id: Iafe9e4c3eb4d32a52a6a1479aa9a2b295cd4ad6f