Clean up access of display content

- ActivityRecord and Task get their own display instead of from
  parent. Because the hierarchy should guarantee the display
  instances are the same.
- Remove redundant getDisplay().mDisplayContent since there is
  no ActivityDisplay after hierarchy unifying.
- Access mDisplayContent directly because WindowContainer#reparent,
  setParent, onDisplayChanged never pass null DisplayContent.
  Currently the only place that clears mDisplayContent is in
  Task#onParentChanged. So the nullity check of mDisplayContent
  only exists in Task or other level's onConfigurationChanged if it
  is called the first time when adding to parent without display.
- The best practice of handling non-attached container might be
  to check at upper entry, e.g. ActivityRecord.isInStackLocked, or
  use WindowContainer#isAttached. Instead of checking the nullity of
  display content everywhere.

Bug: 163976519
Test: WmTests
Change-Id: Ibf6c1368cfeadcad63ad5040a6166d654b906275