Ensuring that our animation callbacks are not stripped by pro guard.

Bug 20296035
diff --git a/proguard.flags b/proguard.flags
index 83a491d..ee6bfa5 100644
--- a/proguard.flags
+++ b/proguard.flags
@@ -62,3 +62,8 @@
   public int getBrightness();
   public void setBrightness(int);
 }
+
+-keep class com.android.launcher3.AppsContainerRecyclerView {
+  public void setFastScrollerAlpha(float);
+  public float getFastScrollerAlpha();
+}
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