How to open the built in Equalizer programmatically in Android?

The following should work to start the default equalizer Activity:

Intent intent = new Intent(AudioEffect
    .ACTION_DISPLAY_AUDIO_EFFECT_CONTROL_PANEL);

if ((intent.resolveActivity(getPackageManager()) != null)) {
    startActivityForResult(intent, REQUEST_EQ);
} else {
    // No equalizer found :(
}

Spotify does the same basically, haven't checked the others.

The necessity of startActivityForResult() is explained in the docs:

The intent carries a number of extras used by the player application to communicate necessary pieces of information to the control panel application.

The calling application must use the android.app.Activity#startActivityForResult(Intent, int) method to launch the control panel so that its package name is indicated and used by the control panel application to keep track of changes for this particular application.