Clear child fragment manager after it is destroyed.

Bug 26945643

After a child fragment manager has been destroyed, it should not
be reused because it is no longer attached to the host. Clearing
it ensures that it will be recreated when it is needed again when
the fragment has been reattached.

Change-Id: I7e52a42a66d761ab69596fdcdb5b2b2e3b4dcc6c