Manually control the animation LS => bouncer when dragging

Set the animator to null instead of using the
default animator. The animation wasn't being properly set back to
the lockscreen when the bouncer dragging was quickly abandoned
since the default animator was being used instead of the
manual animation.

Test: manual
Test: atest KeyguardTransitionScenariosTest
Fixes: 299172035

Change-Id: Ic66eb13e7ac0101031622104500d2ebeb85b4032