Hide scroll arrows from Alt+Tab UI

Recent recommendation from UX was to hide scroll arrows from
Alt+Tab UI, as the primary use case is keyboard focused.

Bug: 397975686
Flag: com.android.launcher3.taskbar_overflow
Test: Verify that arrows are visible when KQS is opened from taskbar
      overflow button (when there's enough items to make KQS
      scrollable), and invisible when triggered using Alt+Tab shortcut.
Change-Id: Iaab234aca94d262b8151a680a08d10d6b39054ed