Android display Splash-Screen while loading

You should not be creating a new thread on startup, instead you should create a view that does not have to wait for your resources to load, as detailed in this article: Splash Screens the Right Way.

As stated in the article, you should create a layer-list drawable instead of a layout XML file:

<layer-list xmlns:android="http://schemas.android.com/apk/res/android">

    <!-- Fill the background with a solid color -->
    <item android:drawable="@color/gray"/>

    <!-- Place your bitmap in the center -->
    <item>
        <bitmap
            android:gravity="center"
            android:src="@mipmap/ic_launcher"/>
    </item>

</layer-list>

Then create a theme using the drawable file as a background. I use the background attribute instead of the windowBackground attribute as suggested in the article, because background takes the status and navigation bars into account, centering the drawable better. I also set windowAnimationStyle to null so that the splash screen does not animate the transition to the MainActivity:

<resources>

    <!-- Base application theme -->
    <style name="AppTheme" parent="Theme.AppCompat.Light.DarkActionBar">
        <!-- Customize your theme here. -->
    </style>

    <!-- Splash Screen theme -->
    <style name="SplashTheme" parent="Theme.AppCompat.NoActionBar">
        <item name="android:background">@drawable/background_splash</item>
        <item name="android:windowAnimationStyle">@null</item>
    </style>

</resources>

Then declare your theme in the manifest for your SplashActivity:

<activity android:name=".SplashActivity"
    android:theme="@style/SplashTheme">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />

        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>

And finally all you have to do in your SplashActivity is start your MainActivity, and the splash screen will only show for as long as it takes for your app to configure:

public class SplashActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        Intent intent = new Intent(this, MainActivity.class);
        startActivity(intent);
        finish();
    }
}

If there are no specific constraints about the time the splash screen should be shown, you could use the AsyncTask in the following way:

public class SplashScreen extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_startup);
        startHeavyProcessing();

    }

    private void startHeavyProcessing(){
       new LongOperation().execute("");
    }

    private class LongOperation extends AsyncTask<String, Void, String> {

        @Override
        protected String doInBackground(String... params) {
            //some heavy processing resulting in a Data String
            for (int i = 0; i < 5; i++) {
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    Thread.interrupted();
                }
            }
            return "whatever result you have";
        }

        @Override
        protected void onPostExecute(String result) {
            Intent i = new Intent(SplashScreen.this, MainActivity.class);
            i.putExtra("data", result);
            startActivity(i);
            finish();
        }

        @Override
        protected void onPreExecute() {}

        @Override
        protected void onProgressUpdate(Void... values) {}
    }
}

If the resulting data if of another nature than a String you could put a Parcelable Object as an extra to your activity. In onCreate you can retrieve the data with:

getIntent().getExtras.getString('data');

How about, in the interest of simplicity, you combine your splash activity with your main activity? That way you get the best of both worlds, namely a splash screen while your data is preparing the first time, and a quick startup when it's been prepared previously. Making the user wait for nothing is not really good form...

Something like:

public class MainActivity extends Activity implements OnClickListener, MediaController.MediaPlayerControl {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        // Initially shows splash screen, the main UI is not visible
        setContentView(R.layout.activity_main);  

        // Start an async task to prepare the data. When it finishes, in
        // onPostExecute() get it to call back dataReady()
        new PrepareDataAsyncTask(this).execute();

    }

    public void dataReady() {
        // Hide splash screen
        // Show real UI
    }

}