Support hierarchy viewer commands via DDM

Hierarchy Viewer currently interfaces to the host via a socket
opened by ViewServer which resides in the WindowManagerService.
Since this has access to all windows, it is enabled only on
debug builds.

This CL adds necessary support to DDM to handle all the commands
required for Hierarchy Viewer. It only misses two commands that
are sent to the Window Manager (which we don't have access to
from the applications).

A future CL will remove the ViewServer functionality.

Change-Id: I1dae316a00737b0cae4e640ccc97bf9bb1d05973