Add a new widget feature value, configuration_optional

If configuration_optional is present, the widget provides a default
configuration. The host may decide not to launch the provided
configuration activity.

Design doc: go/deferrable-widget-config

Test: Successfully built blueline-userdebug. Since this CL only adds
a new value for an existing field, no unit test will be added at this
point. Once we introduce unit tests for widget configurations xml, we
will add related unit tests.

Bug: 177977976
Change-Id: I507cba0b9b8900f9b9616d76c48eb0ed9c50ed72