Fix the root task of secondary split screen could be overwritten.

When external display removed, the remaining tasks should be
reparent to correct parents depends on the status of default
display, if secondary split screen exists, reparent it to the
root task instead of display area.

Bug: 158067169
Test: atest MultiDisplayPolicyTests
Test: after test complete, verify the root of home task doesn't
remain in secondary-split-screen.

Change-Id: Ic4b836263f9acadad2641a19bd369a44aa806d96