[Divider] Fix veil color

Read the activity window background color when showing the veils
(instead of when updating the divider properties) to ensure that
the veil colors are up to date.

Bug: 343523804
Test: atest SplitControllerTest DividerPresenterTest and manual
Change-Id: I2ad3869a9e3b82e85fb31fd1a80e2ba8831dca59
2 files changed