Prevent setting negative alpha for dim animations.

When apply in the dim animation is called, the currentPlayTime value can
be greater than the total duration. If that's the case, the alpha
value would be set to a negative value. Instead, if the currentPlayTime is
greater than duration, just set the final alpha value.

Change-Id: I77b7c512cc8832c2a31e20d49c14550da2cff7cd
Fixes: 73152600
Test: No more flashing dim