Use bottom stage bounds in determineHandlePosition.

Fixes an issue where bottom split task still would get a status bar
input layer due to determineHandlePosition never actually calculating a
position below the status bar. This resulted in top-split handles
opening the handle menu on bottom-split tasks.

Bug: 380879525
Bug: 343561161
Test: Manual
Flag: EXEMPT bugfix
Change-Id: I471ea378fbc3f8affd14641883e44f5ee182a3b4