Clear preferred focusable task when finishing an activity.

...when the task is also going to be removed.
The DisplayArea will have the next preferred focusable task when the
next activity is started.

Flag: com.android.window.flags.polish_close_wallpaper_includes_open_change
Bug: 421436197
Test: atest AssistantStackTests DreamManagerServiceTests KeyguardTests
Change-Id: I2a64533e5cec90727e9436d6c531f3de4b843aa4