I'm pretty sure that you can't achieve that behaviour using only XML files, but if you extend the TextView class to a custom one that capitalises the first letter, then you should be able to use only XML from this point on.
public class CapitalizeTextView extends TextView {
    // Create here constructors matching super
    @Override
    public void setText(CharSequence text, BufferType type) {
        StringBuilder builder = new StringBuilder(text);
        builder.setCharAt(0, Character.toUpperCase(builder.charAt(0)));
        super.setText(builder.toString(), type);
    }
}
From this point on, you only have to use CapitalizeTextView instead of TextView and the first letter will be capitalised. It works from XML or Java/Kotlin.
<mobile.com.capitalize.CapitalizeTextView
    ......
    android:text="not capital letter" />
textView.setText("not capital letter");