Add positionChildAt method to WindowContainer

Added method to change the position of a child among siblings.
It accepts int value, which can either specify a target position
or POSITION_TOP/POSITION_BOTTOM.
When child is moved to top or bottom, there is an option to also
perform same action on parents. This will effectively move the
entire branch of the hierarchy tree to top/bottom.

Test: bit FrameworksServicesTests:com.android.server.wm.WindowContainerTests
Test: #testPositionChildAt
Test: #testPositionChildAtIncludeParents
Test: #testPositionChildAtInvalid
Test: bit FrameworksServicesTests:com.android.server.wm.TaskStackContainersTests
Test: bit FrameworksServicesTests:com.android.server.wm.TaskStackTests
Change-Id: I6ade787487055f1c9a305afea64270c243196614