[Divider] Read veil color from DividerAttributes

If the veil color is configured (non-transparent) in DividerAttributes,
we use the configured color. Otherwise, we use the window background
color of the top activity in the container. Alpha is ignored.

Bug: 293654166
Test: atest DividerPresenterTest
Flag: com.android.window.flags.activity_embedding_interactive_divider_flag
Change-Id: Idc05eb5f249720f4a2471e209f7957755a39fc6b