Add default activity transition class ActivityTransitionAnim.

Author: Sam1301

app/src/main/java/com/zulip/android/util/ActivityTransitionAnim.java

@@ -0,0 +1,22 @@
+package com.zulip.android.util;
+
+import android.app.Activity;
+import android.content.Context;
+
+/**
+ * Default Activity transition animation.
+ */
+
+public class ActivityTransitionAnim {
+    /**
+     * All usages must pass an activity context to execute a transition.
+     *
+     * @param activityContext {@link Context}
+     */
+    public static void transition(Context activityContext) {
+        // avoid invalid class cast exception
+        if (activityContext instanceof Activity) {
+            ((Activity) activityContext).overridePendingTransition(android.R.anim.fade_in, android.R.anim.fade_out);
+        }
+    }
+}