title: cannot find symbol BR in android data binding
body: <p>I want to use Android Data Binding and has configured according to the official document.</p>
  
  <p>the error message is :
  error: cannot find symbol import net.juude.droidviews.BR;</p>
  
  <p>the top level <code>build.gradle</code> file is:</p>
  
  <pre><code>buildscript {
      repositories {
          jcenter()
      }
      dependencies {
          classpath "com.android.tools.build:gradle:1.3.0-beta2"
          classpath 'me.tatarka:gradle-retrolambda:3.0.1'
          classpath 'com.neenbedankt.gradle.plugins:android-apt:1.4'
          classpath "com.android.databinding:dataBinder:1.0-rc0"
      }
  }
  allprojects {
      repositories {
          jcenter()
          mavenCentral()
      }
  }
  </code></pre>
  
  <p>and the build.gradle file in app module is</p>
  
  <pre><code>apply plugin: 'com.android.application'
  apply plugin: 'me.tatarka.retrolambda'
  apply plugin: 'com.neenbedankt.android-apt'
  apply plugin: 'com.android.databinding'
  
  
  android {
      compileSdkVersion 22
      buildToolsVersion "22.0.1"
  
      lintOptions {
          abortOnError false
      }
  
      defaultConfig {
          applicationId "net.juude.droidviews"
          minSdkVersion 15
          targetSdkVersion 22 
      }
      compileOptions {
          sourceCompatibility JavaVersion.VERSION_1_8
          targetCompatibility JavaVersion.VERSION_1_8
      }
  
      buildTypes {
          release {
              minifyEnabled false
              proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.txt'
          }
      }
      retrolambda {
          println("JAVA_HOME: " + System.getenv("JAVA_HOME"))
          println("JAVA7_HOME: " + System.getenv("JAVA7_HOME"))
          println("JAVA8_HOME: " + System.getenv("JAVA8_HOME"))
          javaVersion JavaVersion.VERSION_1_7
      }
  }
  
  dependencies {
      compile project(':ptr-lib-debug')
      compile project(':library')
      compile 'com.android.support:support-v4:22.1.0'
      compile 'com.mcxiaoke.volley:library:1.0.+'
      compile files('libs/droidmocks.jar')
      compile 'com.android.support:support-annotations:22.1.1'
      compile 'com.facebook.fresco:fresco:0.5.0'
      compile 'com.android.support:recyclerview-v7:22.1.1'
      compile 'io.reactivex:rxandroid:0.24.0'
      compile 'io.reactivex:rxjava:1.0.6'
      compile 'com.google.dagger:dagger:2.0'
      apt 'com.google.dagger:dagger-compiler:2.0'
  }
  </code></pre>
  
  <p>the layout file is </p>
  
  <pre><code>&lt;layout xmlns:android="http://schemas.android.com/apk/res/android"&gt;
      &lt;data&gt;
          &lt;variable name="user" type=" net.juude.droidviews.databinding.User" /&gt;
      &lt;/data&gt;
      &lt;LinearLayout
          android:orientation="vertical" android:layout_width="match_parent"
          android:layout_height="match_parent"&gt;
  
          &lt;TextView
              android:id="@+id/user_name"
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:text="@{user.mName}"/&gt;
  
          &lt;TextView
              android:id="@+id/user_sex"
              android:layout_width="wrap_content"
              android:layout_height="wrap_content"
              android:text="@{user.sex}"/&gt;
  
      &lt;/LinearLayout&gt;
  &lt;/layout&gt;
  </code></pre>
  
  <p>the fragment is</p>
  
  <pre><code>package net.juude.droidviews.databinding;
  
  import android.databinding.DataBindingUtil;
  import android.databinding.ViewDataBinding;
  import android.os.Bundle;
  import android.support.annotation.Nullable;
  import android.support.v4.app.Fragment;
  import android.view.LayoutInflater;
  import android.view.View;
  import android.view.ViewGroup;
  
  import net.juude.droidviews.BR;
  import net.juude.droidviews.R;
  
  /**
   * Created by juude on 15-6-9.
   */
  public class DataBindingFragment extends Fragment{
      @Nullable
      @Override
      public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
          ViewDataBinding dataBinding = DataBindingUtil.inflate(inflater, R.layout.fragment_data_binding, container, false);
          //dataBinding.setVariable(BR.)
          User user = new User();
          user.setSex("男");
          user.setName("Juude");
          dataBinding.setVariable(BR.user, user);
          return dataBinding.getRoot();
      }
  }
  </code></pre>
  
  <p>since I did not get syntax error in Android Studio， I can assume that the Android Studio can recognize the BR class;</p>
  
  <p>But when I compile the project, It tells me that 
  <code>error:cannot find symbol class BR</code>.</p>
  
  <p>so I want to know what should I do to make it generate the right BR.java file?</p>