Fixes NPE when getting the source task

The source task was reused when starting activity. However, the source
activity was also removed from the task since the FLAG_ACTIVITY_CLEAR_TASK
was set.

Bug: 443007882
Test: steps on the bug
Test: wm presubmit
Flag: EXEMPT BUGFIX
Change-Id: I5143a65e772a4819a185175ce0b9fb02bb8fc884