Call WC#onDisplayChanged in setParent

WC#mDisplayContent will update when received from WC#onDisplayChanged.

Currently, for Task#mDisplayContent, will update when its stack position change,
or the Task reparented to new display,
but for new activity launch flow, it won't receive onDisplayChanged callback
for the Task,

because before the Task added the stack, the stack has already called
onDisplayChanged when it created and added into TaskStackContainer.

The fix is add a check in WC#setParent to see if the display of new parent is
is same as the current one, if not, calling onDisplayChanged to update
the display from the new parent.

Also, add a unit-test to test this scenerio.

Bug: 142617871
Test: atest WindowContainerTests#testOnDisplayChanged TaskTests \
      TaskStackTests

Change-Id: I84c91dc63508b270a3576335bc9c8a005b72eba6