Allow stacks to hold tasks on various sizes.

Tasks are now resizeable just like stacks. Adjusting the size
of a stack will also adjust the size of all it's containing
tasks. This model allows us to be more flexible
in using stacks as workspaces (like you would have in a
desktop environment) and tasks as the resizeable windows
within the workspace.

Also added "adb shell dumpsys window visible-apps" for
getting the list of visible app windows.

Bug: 22068114
Bug: 19182363

Change-Id: I5bf77063252a5abae4e327f6fc42e607091749f9