new two-pane layout

New, simplified 2-pane ViewGroup.
This version shuffles three panes around, like the last one, but
also supports other floating children like any FrameLayout
would. The animation is now done using ViewPropertyAnimator.
The cross-fade of the conversation list is now done with a
separate "copy" view (ConversationListCopy) that's animated in
tandem with the real list view.

Bug: 6963808
Change-Id: Icee580f663a50537874c1f98e9bbac822f58e571
8 files changed