Make DeskopTaskView use latest TTV

- Made TaskThumbnailViewDeprecated nullable in TaskContainer, with non-null getter that requires feature flag to be on/off
- Removed TaskThumbnailViewDeprecated.setTaskOverlay, as it'll now be null with feature flag on
- Removed TaskThumbnailViewDeprecated from DesktopTaskView xml
- Simplifed DesktopTaskView binding logic, to always get thumbnailViews from viewPool, removeView and recycle all thumbnailViews in onRecycle
- Didn't implement view pooling of TaskThumbnailView due to difficulty with TaskContainer not being recycled togetehr with TaskThumbnailView

Bug: 338360089
Test: TaskThumbnailViewModelTest
Test: manual testing for DesktopTaskView for both enable_refactor_task_thumbnail on and off
Flag: com.android.launcher3.enable_refactor_task_thumbnail
Flag: com.android.window.flags.enable_desktop_windowing_mode
Change-Id: I38a6dfc6bc561689578d1660794f91d30bad4a68