Fallback to endRelOffset for activity surface offsets

We intentionally report incorrect information about activity
bounds to shell for letterbox situations. Until we resolve the
root issue, use endRelOffset as a fallback to keep app-compat
activities from animating in the wrong location.

Bug: 332690307
Test: go activity->activity with letterboxed activities.
Change-Id: I88146b4f8858ee846956b27d3706e719076a4e8c