Polish transition between desktop mode and overview

Fixing desktop task in place along the x-axis and animating its background's alpha to more smoothly handle the transition.

Flag: com.android.launcher3.enable_desktop_task_alpha_animation
Fixes: 320307666
Test: swiped to and from desktop mode and overview
Change-Id: I38d62d0c966733f96c2f4d4ecfa4a9418039fd48