Fix blank recent tasks thumbnails.

When user navigate back to Overview, getAllTaskData will be called and refresh the list of tasks in the repository which cleans the pre-loaded thumbnails and icons from the previous iteration.

In some cases, this causes the thumbnails to be blank in the recents. When getAllTaskData is called after the taskRequest is completed, the tasks will end up in an invalid state without the thumbnail and icon until the user scrolls through the recent apps list or re-open Overview.

Fix: 380067701
Flag: com.android.launcher3.enable_refactor_task_thumbnail
Test: TasksRepositoryTest
Change-Id: Ia15d905a2f5ceb71b87c0b91028ce88894524806