Disable fetching of thumbnails when using snapshots.

- We don't keep the snapshots on the SysUI side so this is just a wasted
  call to load them (the rest of the metadata can still be prefetched
  though).

Test: Launch new tasks and ensure that we aren't fetching task snapshots
      without entering overview.

Change-Id: I1fe01a162f15e4f75b333e7e1bbe32d07e5d0b8d