Removing AM.getTasks call on UI thread during touch down

We maintain a cached task queue using TaskStackChangeListener.
It keeps the top RunningTaskInfo partially in sync with platfrom:
   taskInfo is only initialized when the top task changes
   but not when there are changes within the task itself.

Bug: 214581894
Bug: 220742398
Test: Manual and presubmit
Change-Id: Ifafde574040c03435b78f044350c45f3b49c4bcb