Move add/remove/findPref to ProgressiveDisclosureMixin.

This makes ProgressiveDisclosureMixin the authority for adding/removing
preferences so caller doesn't need to figure out if a preference is on
screen or collapsed.

Bug: 32255863
Test: make RunSettingsRoboTests -j40
Change-Id: I9bd69661b78efd4bb4913665f6ea09f6bdc231f5