Generate new task IDs in tests based on current running tasks instead of hardcoding values.

The fixed task id in tests can cause a collision when generating multiple tasks in the same test case. This is fixed by generating new IDs from the existing tasks.

Fix: 430032085
Test: atest DesktopTasksController
Flag: EXEMPT Fixing Tests
Change-Id: I6c6c7b1222601a199515a088e3cee6727adabeaa