Fix bug with TAPL tests not recognizing full height/width of split tasks

Previously, OverviewTask was only using one task tile to calculate its visible bounds. In the case of split tasks, this sometimes returned a wrong value.

Fixed by changing getVisibleHeight() to calculate height differently if the task is split.

Bug: 307853547
Test: Tests still pass, upcoming changes (ag/25143701) are unblocked.
Flag: N/A
Change-Id: I78221bafd8069275987c310ab05e0ed9e9cd4aa7