Logspam: Properly handle optional app widgets service

If a service is not present and an exeception is thrown in
the SystemServiceRegistry, we log a message and proceed with
the null service. Optional feature services, like app widgets,
are expected to be missing if not supported. Properly
handle the lookup, so no exception that pollutes the logs is
emitted for what is normal operation.

Test: No exception logged with the app widgets feature disabled

Upstream from Meta

Change-Id: I1cae4f9188ec47b55d68d7c8208f0da1c6c263b4