Refactoring TAPL functions for OverviewTask

This CL refactors some methods in OverviewTask to rely on OverviewSplitTask. It also changes the BaseOverview getTasks function to retrieve the parent task and support cases where only the bottomright_snapshot is available in the recent tasks.

Bug: 320633351
Test: TaplTestsSplitscreen
Flag: N/A
Change-Id: I226f895810ce3b46cc107f76cd85e2918abf7088