Ensure root illustration container is also marked as important for accessibility

- The root of illustration_preference layout is not marked as important
  for accessibilty and as a result the descendants can't be focused for
  switch/voice access

Fixes: 319749921
Flag: EXEMPT bugfix
Test: Navigate the gesture settings page with talkback/switch/voice access
      and ensure the lottie image is focusable
Change-Id: I7f9fd53ce1b245b7b07f4e081b9ac708e6be4fe3