[1/N] WindowDecorViewHost: Add viewhost/supplier interfaces and default impls

Spiritual revert^2 of Iae7589879d56d802c85b8c4a72a8dcc9b20d6afb, but
broken down into smaller changes.

Creates an abstraction for creating, updating and releasing the surface
and view hierarchy drawn by the WindowDecoration (aka the caption bar).
Also provides a default implementation that supports rendering the view
hierarchy synchronously and asynchronously.

Note that these abstractions aren't used yet, so no behavioral or
performance changes are expected with this CL alone.

Bug: 360452034
Flag: EXEMPT refactor
Test: m && atest WMShellUnitTests

Change-Id: Ic6866a5575e736ef5877de52a6a8787a28c74c19