Avoid adding unnecessary topMargin to TaskThumbnails in DesktopTaskView

- TaskThumbnails in DesktopTaskView already take into account topMarign during onMeasure, we should avoid adding double margin
- mTaskThumbnailViewDeprecated's visibility is always GONE, so no need to set margin either

Fix: 336968808
Test: manual, image test to follow-up
Flag: ACONFIG com.android.window.flags.enable_desktop_windowing_mode DEVELOPMENT
Change-Id: I6a7cb128ead0b042974b95199ab720cafc3bcc18