Use onTaskThumbnailChanged as sourced of truth for thumnbail override

- Both RecentsView.screenshotTask and RecentsView.updateThumbnail uses thumbnail that comes from recentsAnimationController.screenshotTask. recentsAnimationController.screenshotTask always results in TaskVisualsChangeListener.onTaskThumbnailChanged, which is a better source of truth to rely on rather than having 3 entry points for thumbnail override
- As a thumbnail objects from onTaskThumbnailChanged has the same snapshotId but are different instances, updated RecentsViewModel.waitForThumbnailsToUpdate to reflect that

Bug: 342560598
Test: RecentsViewModelTest
Flag: com.android.launcher3.enable_refactor_task_thumbnail
Change-Id: I79b6e32224df4192e8df6930d87b222ed93bf8d8