Fix UDFPS flicker on LOCKSCREEN->DOZING

The UDFPS background alpha must be set to 0f when in dozing
state. However, it should not animate from its current value.
Allow null values in onStep() return to indicate no change is
needed, and make sure the alpha gets to 0f onCancel or onFinish.

Fixes: 363899639
Test: atest KeyguardTransitionAnimationFlowTest
Test: manual look from flicker on LOCKSCREEN->DOZING
Test: manual ensure pulsing notifications do not show UDFPS
background
Flag: EXEMPT bugfix

Change-Id: Ic956f78db206d5d8d7e227be2b75bf51ab37cacc