Throw exception if RemoteCollectionItems contains null entry

During the adapter conversion process, it is possible that null
RemoteViews would be passed by the factory. Since entries in
RemoteCollectionItems should be non-null, we will throw an exception
when applying the collection to the views.

Test: Presubmit
Flag: android.appwidget.flags.remote_adapter_conversion
Fix: 403645464
Change-Id: I400217a183c970ecd3348d391f16fba1c41c557e