Fix split-tasks & wallpaper flicker when unlocking keyguard

As now we have KeyguardTransitionHandler to handle keyguard animation
and takes over other mixed transition cases in
DefaultMixedHandler#animateKeyguard, 2 things needs to be fixed:

1) All leashes handled by KeyguardTransitionHandler will by default
   reparented to the transition root which the position (0,0) is screen
   space based, so in KeyguardUnlockedAnimationController we have to
   change using screenSpaceBounds position for all leash in case seeing
   split-tasks animating on the same position.

2) In KeyguardService, we have to ensure to only apply the opening
   root task & wallpaper setting alpha 0 for fade-in animation. In case
   seeing the flicker that previously we didn't consider that.

Note that setting alpha 0 for opening wallpaper targets is just fixing
one of the wallpaper flicker source for Bug 283465374 and the CL is not
intended to fully fixing all wallpaper flicker cases.

Fix: 282593683
Bug: 283465374
Test: manual as issue steps
    1. Launch Gmail and Chrome in split screen.
    2. Turn off the screen by pressing power button.
    3. Turn on the screen by pressing power button.
    4. Observe the split screen transition without any flicker
Change-Id: I13070a9e6f327bc1f3cb4f037b411a0c43c90250