Override displacement for transient taskbar instead of mCurrentShift

mCurrentShift is based on displacement, but is used and set in different
places. To keep consistency throughout, we should override the
displacement at the source instead.

Test: manually swiped up between home/overview threshold and catch up
threshold; let go to go to overview and ensure no jump occurs.
Flag: teamfood (ENABLE_TRANSIENT_TASKBAR)
Fixes: 259981285

Change-Id: Id9f5b2bcdc2e56e15ddb6d93c4ee5d5ececc094d