Attach the background-panel to DisplayContent.

The background-panel is not attached to the DisplayContent,
therefore the parent of its parent, BackgroundWindowManager,
is null. This will result in the failure to record the screen
and black screen.

Therefore, we need to attach the background-panel
to the displaycontent.

bug: 254436057
Test: atest BackgroundWindowManagerTest
Test: manual record screen and trigger one-handed mode

Change-Id: I440feb76fdc59d73339770bba57857efa828c7d2
Signed-off-by: xinying <xinying1@xiaomi.corp-partner.google.com>