Don't animate clock when turning screen off.

This change stops the animation because there isn't a
transition from the no-header state to the music header
state when turning the screen off (to AOD). Since there
isn't a transition, there isn't an animation.

This assumes that there isn't a transition from unlocked to
lock screen. If there is, then there would be an animation
of the music going away while arriving at the lock screen.

Fixes: 137383007
Test: Checked repro steps in bug, clock doesn't animate.
Test: Also checked repro steps when audio is paused, clock doesn't
animate.
Test: atest KeyguardSliceProviderTest.java

Change-Id: If39777340b72bc623d6690bc4f784c7f5c26ea8d