Thread safety support for variable callbacks (#406)

Author: milos1290

AndroidSDKCore/src/main/java/com/leanplum/Var.java

@@ -350,7 +350,7 @@ private void cacheComputedValues() {
   /**
    * Updates variable with values from server.
    */
-  public void update() {
+  public synchronized void update() {
     // TODO: Clean up memory for resource variables.
     //data = null;
 

AndroidSDKCore/src/main/java/com/leanplum/callbacks/VariableCallback.java

@@ -36,7 +36,9 @@ public void setVariable(Var<T> variable) {
   }
 
   public void run() {
-    this.handle(variable);
+    synchronized (variable) {
+      this.handle(variable);
+    }
   }
 
   public abstract void handle(Var<T> variable);