Fix testUpdateDragShadow_detachedView for car multi-tasking devices

On car multi-tasking devices, the test activity doesn't take the full
screen. So test fails on wrong assumption.

Use Activity location, instead of display size for drag and drop.

Fix: 330770221
Test: atest android.view.cts.ViewTest#testUpdateDragShadow_detachedView
Change-Id: I5ff9050c962b5d4f1f10ade4e91d3231d7dc0c0c