Move caption under DecorationContainerSurface

Therefore caption can be put at the same layer as other window
decorations. It adds CaptionContainerSurface to host the caption so that
WindowDecoration can put the caption at the same position as the task
surface.

Bug: 241598994
Test: atest WindowDecorationTests
Test: Caption is shown in front of embedded activities.

Change-Id: I9ec571859680f622449f4c3b5cea27222ec4c788