Apply drag elevation to bubble bar container

While dragged, bubble bar needs elevation to show up above dismiss view
and dismiss view scrim.
Previously, bubble bar and dismiss view were under the same parent, drag
layer. This allowed us to position bubble bar above dismiss view by
adding elevation to it.
After introducing the bubble bar container, bubble bar is not shown on
top of dismiss view when its elevation changes. Because they are no
longer under the same parent.
Move drag elevation from bubble bar to bubble bar container. The
container and dismiss view are under the same parent.

Bug: 376140375
Test: manual, drag bubble bar or bubble, check they are above dismiss
  view and the scrim
Test: receive a new message, check that flyout is shown
Flag: com.android.wm.shell.enable_bubble_bar
Change-Id: Ie0fb677929bb2413c10b934fc8b362cde7b8cecd