SF: Introduce LayerHierarchy and LayerHierarchyBuilder

LayerHierarchy allows us to navigate the layer tree in
z-order, or depth first traversal. The hierarchy is created
from a set of requested layer states and the structure
itself does not contain additional layer states.

This is in contrast to the existing model where the Layer
class contained information about the hierarchy it belonged to.
By breaking out of this model, we can make the layer class
(now called RequestedLayerState) simpler and separate client
states from the hierarchy.

This also allows us to construct more flexible hierarchies that
handles mirroring and relative parents more efficiently.

When traversing the hierarchy, each node can be visited multiple
times. For example, it could be visited as a child, a relative child,
or a mirrored child. The class introduces the concept of variants
to distinguish the traversal path.

In the upcoming cl, this traversal will be used to compute the final
composition state in z-order.

Bug: 238781169
Test: presubmit
Change-Id: I9902e521df2d47556e3a671e2134d5be8cd2a73f