Use top sched group for last non-top freeform app

To make desktop-like environment more responsive. Such as the freeform
tasks can be put like split screen, then both side are important.

Bug: 200769420
Test: MockingOomAdjusterTests#testUpdateOomAdj_DoOne_VisibleActivities
Test: Launch 2 freeform apps.
      adb shell dumpsys activity o | grep "T/A/TOP"
      The output will show 2 records.
Flag: com.android.window.flags.process_priority_policy_for_multi_window_mode

Change-Id: If44a4ae793875ebb0885feb25e397ff57699a203