Fix text behind button issue on smaller screens

On a Pixel 2, the text is behind the top button. Reducing the top margin. 

Also, center the initial text.

Author: jkurtw

app/src/main/res/layout/activity_main.xml

@@ -68,9 +68,10 @@
       android:id="@+id/textview_first"
       android:layout_width="wrap_content"
       android:layout_height="wrap_content"
-      android:layout_marginTop="180dp"
+      android:gravity="center"
+      android:layout_marginTop="90dp"
       android:text="@string/placeholder_txt"
       app:layout_constraintEnd_toEndOf="parent"
       app:layout_constraintStart_toStartOf="parent"
       app:layout_constraintTop_toTopOf="parent" />
-</androidx.constraintlayout.widget.ConstraintLayout>
+</androidx.constraintlayout.widget.ConstraintLayout>