Of course, you can always set the gravity of the original text view. This allows you to not have to worry about formatting and padding.
For example
AlertDialog.Builder builder = new AlertDialog.Builder(this);
builder.setTitle("Title");
builder.setMessage("Message");
builder.setPositiveButton("OK", null);
AlertDialog dialog = builder.show();
// Must call show() prior to fetching text view
TextView messageView = (TextView)dialog.findViewById(android.R.id.message);
messageView.setGravity(Gravity.CENTER);
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