Fix asset loading on ResourcesImpl swap

The ResourceLoaderManager reference was not being updated and
so any configuration change would drop loaders from the asset
search list.

Also re-enables and updates a test originally designed to catch
this kind of problem.

Test: atest ResourceLoaderChangesTest

Change-Id: I5d9bba2f85c516573d3da0c8f1a97a1d716b3a34