Use top sched group for apps in split screen

Currently only one side has the top sched group. This change makes
both sides of split have top priority. This might reduce frame
drop if the non-top side is running a video conference.

Bug: 200769420
Test: MockingOomAdjusterTests#testUpdateOomAdj_DoOne_VisibleActivities
Flag: com.android.window.flags.process_priority_policy_for_multi_window_mode

Change-Id: I6a842810c023648b206f4c9fdd2db403dce20b91