Clear hashed gradient objects when a new value callback is set. (#1639)

Fixes #1608
diff --git a/lottie/src/main/java/com/airbnb/lottie/animation/content/GradientFillContent.java b/lottie/src/main/java/com/airbnb/lottie/animation/content/GradientFillContent.java
index 869cb4e..740819d 100644
--- a/lottie/src/main/java/com/airbnb/lottie/animation/content/GradientFillContent.java
+++ b/lottie/src/main/java/com/airbnb/lottie/animation/content/GradientFillContent.java
@@ -256,6 +256,8 @@
          colorCallbackAnimation = null;
        } else {
          //noinspection rawtypes
+         linearGradientCache.clear();
+         radialGradientCache.clear();
          colorCallbackAnimation = new ValueCallbackKeyframeAnimation<>(callback);
          colorCallbackAnimation.addUpdateListener(this);
          layer.addAnimation(colorCallbackAnimation);