Make windowing limit more robust to unrecognized tasks

Sometimes we can end up with tasks in the DesktopRepo that don't exist /
that we cannot fetch a TaskInfo object for, with this CL we ignore such
tasks and try to minimize the next task from the bottom (instead of not
minimizing any task).

Bug: 414376840
Flag: com.android.window.flags.enable_desktop_task_limit_separate_transition
Test: DesktopTasksLimiterTest
Change-Id: Ic75c659b58d96679e5621d295e8d0045b5e6d74b