Restrict reflection access to ValueAnimator's sDurationScale

An alternative method, areAnimatorsEnabled() can be used to determine
if animations are turned off or not. Otherwise, reflection access of the
value are not permitted since it is an implementation detail that can
break battery saver mode or user's choice of disabling system-wide
animations.

Bug: 123767751
Test: n/a
Change-Id: I2146613a5921e229f7db641c34732bc1044766fc