Make LottieDrawable public again

Fixes #93
diff --git a/lottie/src/main/java/com/airbnb/lottie/LottieDrawable.java b/lottie/src/main/java/com/airbnb/lottie/LottieDrawable.java
index 4e14c55..c86ba17 100644
--- a/lottie/src/main/java/com/airbnb/lottie/LottieDrawable.java
+++ b/lottie/src/main/java/com/airbnb/lottie/LottieDrawable.java
@@ -23,7 +23,7 @@
  * handles bitmap recycling and asynchronous loading
  * of compositions.
  */
-class LottieDrawable extends AnimatableLayer {
+public class LottieDrawable extends AnimatableLayer {
   private LottieComposition composition;
   private final ValueAnimator animator = ValueAnimator.ofFloat(0f, 1f);