ScrimController: Wait with clearing wakup animation until done

Fixes an issue where if another animation was started during
the wakeupFromDoze animation, the scrim would flash white
because the original animation got canceled. Instead now wait
until the animation is actually completed, not just altered.

Change-Id: I7738ec838d77f78dde2c5810369c50b4fafaeb67
Fixes: 64412754
Test: Trigger assistant from AOD; verify there is no white flash.