Add missing Activity import

Author: swansontec

android/src/main/java/com/cmcewen/blurview/BlurViewManager.java

@@ -1,5 +1,6 @@
 package com.cmcewen.blurview;
 
+import android.app.Activity;
 import android.graphics.drawable.Drawable;
 import android.view.View;
 import android.view.ViewGroup;