Fix the activity windowingmode error of exiting pip

The top activity will keep fullscreen mode, even if it is reparented
to multi-window parent, so we reset the activity windowing mode to
match parent here.

Bug: 317645068
Test: Manual test

Change-Id: I96b7f821ad55565c4c4f1d786522988e0c703ec4
Signed-off-by: buwenjing <buwenjing@xiaomi.com>