Update Backlinks to capture data from all tasks

- In the previous version we were using ActivityManagerService's
  getAllRootTaskInfosOnDisplay API to get all the tasks on the display
  but this didn't return non-focused task from split screen as separate
  task which made it difficult to extract the non-focused tasks
  TaskInfo.
- The current getTasks API from the same service returns all tasks
  separately thus making it possible to get task info of non-focused
  task from split screen apps.

Bug: 300307759
Flag: com.android.systemui.app_clips_backlinks
Test: atest AppClipsActivityTest AppClipsViewModelTest
Change-Id: Ibb88ed83ba3439e175a6749389b497ea8735a823