Polish the transition when unlocking to home

- Only take snapshot if keyguard is secure because the snapshot
  doesn't help too much if there is unlocking window animation,
  because screen is already turned on in this case.
- Only use the snapshot when unlocking to home directly because
  the visual look may be worse with translate animation.
- Dismiss the snapshot starting window directly when the main
  window is ready, so it has less chance to see the fading
  translucent outdated snapshot.

Bug: 140811348
Bug: 148553119
Test: Unlock device with fingerprint and password.
      Check the top area of home doesn't have obvious
      fading scrim.

Change-Id: Ifb1302bd185d83ca9b203cf5d01a8f50fa1d6e69