Use background priority for a freeform task which is fully occluded

When a freeform task is logically visible but it is visually occluded
by other freeform tasks, make it use lower priority to reduce the
contention of CPU with tasks which are truly visible to user.

Bug: 325594711
Flag: com.android.window.flags.bg_priority_for_occluded_freeform_tasks
Test: MockingOomAdjusterTests#testUpdateOomAdj_DoOne_VisibleActivities
Change-Id: Ib90b54de5591af3c9b831740cd5b5ca4f828782c