Fix the state cannot transition to DOZE_PULSING while docking.

If the next ScrimState is requested in a short time (Scrim state changes
from KEYGUARD -> AOD -> PULSING), it has chance mCallback will be reset
to null due to the cancel of previous animator.

Bug: 122943655
Test: Manual.
Change-Id: I1e7c3266021b066ed864b0a867522d69735d2241
1 file changed