Delay taskbar background fade in animation

- Taskbar background will fade in based on
  1. Velocity threshold
  2. End target gesture destination

Bug: 298089923
Bug: 345768019
Test: swipe up fast to go home, note no taskbar bg
      swipe up slow to reveal taskhome, note taskbar bg shows immediately
Flag: com.android.launcher3.enable_scaling_reveal_home_animation DISABLED

Change-Id: I2c16352e1c0c52a8afc49900a39b80383bacde62