Fix NPE in notifyCrossProfileProvidersChanged

The mWidgetProviderListeners is by default null. It is initialized by
addOnCrossProfileWidgetProvidersChangeListener method. And this method
is called by AppWidgetServiceImpl.

On devices without app widget service, e.g. Wear,
mWidgetProviderListeners is null. And calling
notifyCrossProfileProvidersChanged throws NPE.

Flag: EXEMPT BUGFIX
Bug: 436819350
Test: Locally build and test work profile provision
Test: Presubmit

Change-Id: Ic7216edddf5e3564e81da7eefbedb826f7deb421