[2/N] WindowDecorViewHost: move async relayout to RelayoutParams

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

Refactors the sync and async capability from living in
DesktopModeWindowDecoration to being in the base WindowDecoration with
it being controlled by RelayoutParams#mAsyncViewHost.

It also contains small reorganization of methods to prep for migration
to WindowDecorViewHost, such as grouping layout code in #updateViewHost
and #updateViewHierarchy methods.

This is a pure refactor, so no behavior or performance changes are
expected with this CL alone.

Bug: 360452034
Flag: EXEMPT refactor
Test: m WMShellUnitTests
Change-Id: Ia13bd3e915b6bea6fb275e4308940af851d6fe3c