Refine password disappear animation.

In the disappear animation, tie alpha of the view to animated value.
Before this fix, the view alpha is 1 until the animation is complete and
looks janky.

Fixes: 284436144
Test: unlock password with swipe up.
Test: unlock password with tap on notificaiton.
Test: unlock password with tap on settings from QS.
Change-Id: Ide0e632df5f4f32ecdb5ab27d4435f3e3785eecc