Skip task snapshot if display has not been enabled

Otherwise if keyguard changes to be showing when booting
and keyguard secured state has not been updated yet, it
will cause allowSnapshotHome to be true. And then take
snapshot for FallbackHome, that is useless and wasteful.

Bug: 192400758
Test: "adb shell dumpsys window w | grep -A5 SnapshotCache"
      should show zero entry after device is booted.
Change-Id: Ia4afe69a70065fc3972fb9cb3dccc1e4b3b35a79