Block edge back gesture by mouse pointer

Bug: 403094305
Test: Manual, i.e. verified that back gesture is not triggered by
      mouse pointer drag from edge of screen. Also verified that
      three finger trackpad back gesture remains functional
Flag: com.android.systemui.block_mouse_edge_back_gesture
Change-Id: I0afdbeb1921357a982e4d44c968d07f163c6eaf8