[Dividier] Fix divider line issue when dragging near the edge

Before this change, the divider line is a view and its updates are out
of sync with surface updates, causing the divider line position to
misalign with veils while dragging. This CL moves the divider line into
a child surface so that its updates are synced with veils.

Bug: 355056613
Test: atest DividerPresenterTest
Flag: com.android.window.flags.activity_embedding_interactive_divider_flag
Change-Id: Icea0f5eae5266d54a742843548a5026a916b55c1