With the material components library you can use the setAnchorView method to make a Snackbar appear above a specific view.
In your case if you are using a BottomAppBar and a fab, you should define the fab in the setAanchorView.
Something like:
FloatingActionButton fab = findViewById(R.id.fab);
Snackbar snackbar = Snackbar.make(view, "Snackbar over BottomAppBar", Snackbar.LENGTH_LONG);
snackbar.setAnchorView(fab);
The result:

With a BottomNavigationView you can define it as anchorView:
Snackbar snackbar = Snackbar.make(view,"Snackbar over BottomNav",Snackbar.LENGTH_INDEFINITE);
snackbar.setAnchorView(bottomNavigationView);
snackbar.show();
Result:
