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You don't need to know what the default messaging app is in order to launch it. All you need it to send this intent:

    Intent intent = new Intent(Intent.ACTION_MAIN);
    intent.addCategory(Intent.CATEGORY_APP_MESSAGING);
    intent.setFlags(Intent.FLAG_FROM_BACKGROUND | Intent.FLAG_ACTIVITY_NEW_TASK | Intent.FLAG_ACTIVITY_SINGLE_TOP | Intent.FLAG_ACTIVITY_CLEAR_TOP);
    startActivity(intent);

Or if you actually want to start a text message to a specific phone#:

    Intent intent = new Intent(Intent.ACTION_SENDTO, Uri.parse("sms:5551234567"));
    intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);
    startActivity(intent);

Obviously, replace "5551234567" with the actual phone number