Retain loaders through stopped config changes

Framework edition

Previously we would throw away any stopped LoaderManagers when we went
to retain instances to pass along as nonConfigurationInstances during
config changes or similar activity restarts. This causes loaders to do
more work than they need to when a calling activity starts a new
activity on top, a config change happens (e.g. screen rotation) and
then the top activity is finished, restarting the caller in a new
configuration. The loaders would go through onReset unnecessarily,
potentially throwing away data to be reloaded again after the config
change completes.

Instead of throwing away stopped LoaderManagers in this case, restart
them and retain them across the config change so they can resume where
they left off.

Bug 27176186

Change-Id: Ia52c6448d2ad41dcb25d493770d9ffae20a19d2a