Add basic access controls to cc::PropertyTrees

This makes all member fields of PropertyTrees private, and adds basic
getter/setter methods and const correctness. The motivation for this
change is to make property trees safe for non-blocking commit, so some
of the design choices reflect that priority.

The non-const accessors from PropertyTrees to the individual trees are
given _mutable suffixes to force callers to be explicit about whether
they are modifying data, rather than relying on the compiler to pick
the const or non-const version of a method.

This also moves tree indexing out of PropertyTrees and into the
individual trees as PropertyTree<T>::element_id_to_node_index_.

The reserved node id numbers (PropertyTree<T>::kInvalidNodeId, etc.)
are moved out of PropertyTree<T> to a stand-alone non-templated enum;
the templated definitions are redundant. They are also renamed by
inserting 'Property', to avoid collision with cc::kInvalidNodeId
(defined in cc/paint/node_id.h).

Bug: chromium:1255972
Change-Id: Ifd718795a0bc26773c6d64efbed95e05197f1e31
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3426308
Reviewed-by: Ian Vollick <vollick@chromium.org>
Reviewed-by: Vladimir Levin <vmpstr@chromium.org>
Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/main@{#966064}