Add documentation for layer drawing order

This allows control to be delegated to different parts of the system - such as SystemServer,

SysUI and Apps.

### Layer Drawing Order

Layers are drawn based on an inorder traversal, treating relative parents as

direct parents. Negative z-values place layers below their parent, while

non-negative values place them above. Layers with the same z-value are drawn

in creation order (newer on top).  However, relying on creation order for

z-ordering is discouraged; use unique z-values whenever possible for better

control.

Traversal pseudo code:

```

fn traverseBottomToTop(root):

  for each child node including relative children,

    sorted by z then layer id, with z less than 0:

          traverseBottomToTop(childNode)

  visit(root)

  for each child node including relative children,

    sorted by z then layer id, with z greater than

    or equal to 0:

          traverseBottomToTop(childNode)

```

### Layer Lifecycle

Layer is created by a client. The client receives a strong binder reference to the layer

handle, which will keep the layer alive as long as the client holds the reference. The