fix(cdk/listbox): error with multiple preselected values (#25621)

The CDK listbox had a subscription to its model changes in the constructor which fires immediately due to the `startsWith` and which was attempting to sort the selected values when in multiple selection mode. The problem is that the options aren't available until `ngAfterContentInit` which was causing an error when we tried to do the sorting.

These changes move the change event subscription to `ngAfterContentInit` so that options are guaranteed to be defined when it fires.

Fixes #25611.

(cherry picked from commit 1b0265c)

Author: crisbeto