Read DisplayContent's children from rootDisplayArea field

Because we have DisplayContent extend DisplayArea, it no longer
populates the field window_container, but that can be found in
root_display_area.window_container.

The tool will still support old client.

Bug: 156425461
Test: Able to open winscope window traces
Change-Id: I32ed38e8c1e9d77ec813230d5e1f86aecf8e14c5
1 file changed