Adding public API for marking nodes as clusters and sections.

Clusters:
We need clusters to limit the size of the “tab loop” by
widgets related to the current context: working in the
app’s client area, choosing a command in the action bar
etc.

Clusters are a generalization of the current action bar’s
behavior.
An activity can have several clusters.
A cluster is a view or a view group group marked as such.
Pressing Tab loops inside the cluster, but you can exit it via
arrows.
You can teleport between clusters via special key combos.

Sections:
Sections live inside clusters.
They are needed for simplifying navigation in complex
hierarchies: instead of tabbing or arrowing, you can simply
teleport to the the next/previous section by pressing a
special key combo.
Example: think about panes in GMail app or dir/files
panels in a file manager.

Otherwise, sections are normal view groups: for example,
they don’t limit keyboard navigation in any way.

Bug: 32151632
Test: Checking for syntax errors and that Android starts.
Change-Id: Ic78495d0749db65d5177017553d37f870354c6bc