fix(ngOptions): always set the 'selected' attribute for selected options

We don't set selected property / attribute on options that are already selected.
That happens for example if the browser has automatically selected the first
option in a select. In that case, the selected property is set automatically, but
the selected attribute is not. This doesn't impact the functionality of the select,
but it can be problematic if option elements are selected with `option[selected]` in tests.

Closes #14115
Closes #14125