Move child FragmentManager shutdown for retained fragments

Some apps out there do some creative things in terms of calling their
own onDetach methods from elsewhere. Perform the child FragmentManager
shutdown for retained fragments in a package-level perform method
instead of in onDetach itself.

Also mark all existing fragment methods with @CallSuper.

Bug 28293847

Change-Id: I343b610f8d427fe5cabd523a78fbf0e6f3b34c57