Update the stable bounds stored in TaskPositioners on new drag.

Updates the stable bounds on every drag start rather than just the
first. This accounts for display rotation changing the stable bounds.

Bug: 296921896
Test: Manual; drag resize a task, rotate screen, resize again without
old stable bound restrictions.

Change-Id: Ied8e381faeea68eb5078520954ba6c45a09e13bd
4 files changed