Add utilities to persist lists of Persistable objects

This change adds a utility class to enable persistance of Lists of
objects.

The PersistableBundle class does not currently support lists or arrays
of PersistableBundles, presumably due to the potential for key
conflicts. The utility classes added here avoid that concern by
nesting all lists as separate persistable bundles.

Bug: 163594033
Test: New PersistableBundleUtilsTest added, passing
Change-Id: I89478cf0d05d41a4b0d769de4859421061a1f1d9