Offset drag to desktop bounds to start position.

Fixes an issue where split right/bottom position adds an unwanted offset to the drag to desktop animation start by offseting TO the start position rather than BY the start position.

For fullscreen and split left/top tasks, the two are functionally identical due to startBounds position being (0, 0). Stage position being factored in means we need to ignore the current task position.

Bug: 323378048
Test: Manual to verify animation, ABTD to verify no regressions in performance suite
Flag: EXEMPT, minor change
Change-Id: I52d1b1c675e0e319260ee747bcd694753b26ac5c