Add getInterpolator method to Animator

[nhaarman]

Since API level 18, the Animator class contains a method stub for getInterpolator:

/**
 * Returns the timing interpolator that this animation uses.
 *
 * @return The timing interpolator for this animation.
 */
public TimeInterpolator getInterpolator() {
    return null;
}

http://grepcode.com/file_/repository.grepcode.com/java/ext/com.google.android/android/4.3_r1/android/animation/Animator.java/?v=diff&id2=4.2.2_r1