Fix crashes in AppWidgetHost when app_widgets is not supported

Even though an app is supposed to declare android.software.app_widgets
when relying on the widgets API, some apps still use this API without
declaring the feature.  Previously AppWidgetHost relied on having an
AppWidgetService available, without checking for feature development
before dereferencing the service.

This stubs out the AppWidgetHost interface when the app_widgets feature
is not available.

Bug: 28847209
Test: run nova launcher, observe no crash

Change-Id: Idaf46de990fffa6c6a03562f8e097c0a6a320c8a
(cherry picked from commit 0536802defc7df6edb22e076b1147d0d919da0f8)