ColorStateList in android:tint was not supported prior to API 21.
See: https://code.google.com/p/android/issues/detail?id=204671
You can use AppCompat's AppCompatResources and support-v4 DrawableCompat to support pre-lollipop. First, remove android:tint="@color/add_button_tint" from your layout. Then set the ColorStateList programmatically:
java
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.search_button);
ColorStateList csl = AppCompatResources.getColorStateList(this, R.color.add_button_tint);
Drawable drawable = DrawableCompat.wrap(fab.getDrawable());
DrawableCompat.setTintList(drawable, csl);
fab.setImageDrawable(drawable);