Fix issue with PiP touch drift.

- Fix issue with cumulative truncation of positions causing a large x/y
  error causing drift. Instead of using the rounded stack position,
  keep track of the cumulative delta and apply that on each move.

Bug: 62541932
Test: Pip activity, move around with finger and ensure no drift
Change-Id: Ic7737a6b19672ee8a1d98fd57f84a1d89f4a17e9