set FAB anchor gravity dynamically

The CoordinatorLayout.LayoutParams class has the anchorGravity field for that. For example:

FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
CoordinatorLayout.LayoutParams lp = (CoordinatorLayout.LayoutParams) fab.getLayoutParams();
lp.anchorGravity = Gravity.BOTTOM | GravityCompat.START;
fab.setLayoutParams(lp);