pepper: View API refactoring to separate z-order from parent-child relation.

pepper: View API refactoring to separate z-order from parent-child relation.

Sometimes we want parent-child relationship to affect only transform of the
child not z-order. By separating z-order contraint from parent-child
relationship, we can do much more things with the APIs. For the previous style
stacked views, APIs that are changing z-order now take subtree argument
indicating whether entire subtree movement or just the individual view.

Change-Id: I7c50fae2b8dcabb336571f7dc2b93ca5d161e66c