Expose method to clear all visible recent tasks

- Atomically remove all the visible tasks so that SysUI doesn't need to
  remove each task individually.

Bug: 80471073
Test: atest FrameworksServicesTests:RecentTasksTest#testRemoveAllVisibleTasks
Change-Id: I23a6e152e94d5462948ab40adc9d7baf593847e1