Добавьте один макет выше BottomNavigationView
Пример:
<android.support.design.widget.CoordinatorLayout
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/myLayout"
android:layout_above="@id/bottomNavigationView">
</android.support.design.widget.CoordinatorLayout>
, затем в коде Java используйте:
final View viewPos = findViewById(R.id.myLayout);
Snackbar.make(viewPos, R.string.snackbar_text, Snackbar.LENGTH_LONG)
.setAction(R.string.snackbar_action_undo, showListener)
.show();