Clear child FragmentManagers when destroying Fragments.

If Fragment has instantiated a child FragmentManager and is later
detached, it retains its reference to the child FragmentManager which
has been destroyed. This causes an IllegalStateException in the
child FragmentManager if the original Fragment is reattached.

Fixes Issue 42601.

Change-Id: I8db2b1a110a341dc259939723f4c5ec131ca5f1e