Fix scrim offset to (0,0) for letterboxed cross-activity-back

When the cross-activity-back animation was played for a letterboxed app which was placed in the bottom half of split screen, the scrim was wrongly placed in the upper half.

Bug: 328446606
Flag: ACONFIG com.android.systemui.predictive_back_system_anims NEXTFOOD
Test: Manual, i.e. testing back animation in letterboxed app which is placed in the bottom half of split screen
Change-Id: I8fa17deda8daffcb708b07073ac9d378d69a1195