title: ClassNotFoundException for Activity after signing apk
body: <p>While I'm testing the debug version of my app — it's OK. After I sign it for Google Play release — it starts to FC with the following exception:</p>
  
  <pre><code>java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.wiagames.guitartuner/com.wiagames.guitartuner.activities.MainActivity}: java.lang.ClassNotFoundException: com.wiagames.guitartuner.activities.MainActivity
  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1983)
  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
  at android.app.ActivityThread.access$600(ActivityThread.java:130)
  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
  at android.os.Handler.dispatchMessage(Handler.java:99)
  at android.os.Looper.loop(Looper.java:137)
  at android.app.ActivityThread.main(ActivityThread.java:4745)
  at java.lang.reflect.Method.invokeNative(Native Method)
  at java.lang.reflect.Method.invoke(Method.java:511)
  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
  at dalvik.system.NativeStart.main(Native Method)
  Caused by: java.lang.ClassNotFoundException: com.wiagames.guitartuner.activities.MainActivity
  at dalvik.system.BaseDexClassLoader.findClass(BaseDexClassLoader.java:61)
  at java.lang.ClassLoader.loadClass(ClassLoader.java:501)
  at java.lang.ClassLoader.loadClass(ClassLoader.java:461)
  at android.app.Instrumentation.newActivity(Instrumentation.java:1053)
  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1974)
  </code></pre>
  
  <p>Manifest:</p>
  
  <pre><code> &lt;application android:label="@string/app_name"
                   android:icon="@drawable/ic_launcher"&gt;
  
          &lt;activity android:name=".activities.MainActivity"
                    android:label="@string/app_name"
                    android:configChanges="orientation|keyboardHidden"
                    android:theme="@style/AppTheme"&gt;
              &lt;intent-filter&gt;
                  &lt;action android:name="android.intent.action.MAIN"/&gt;
                  &lt;category android:name="android.intent.category.LAUNCHER"/&gt;
              &lt;/intent-filter&gt;
          &lt;/activity&gt;
  
          &lt;activity android:name="com.google.ads.AdActivity"
                    android:configChanges="keyboard|keyboardHidden|orientation|screenLayout|uiMode|screenSize|smallestScreenSize"/&gt;
  
      &lt;/application&gt;
  </code></pre>
  
  <p>How to fix it?</p>