Address API Review comments

- throw NullPointerException if the package name is null; there’s
  already a method with the specified null behavior.
- Return value of this method should be @NonNull - return an empty
  list. Document this.

Test: Unit test on AppWidgetServiceImplTest
$ runtest --path=services/tests/servicestests/src/com/android/server/appwidget/AppWidgetServiceImplTest.java

Bug: 37544056
Change-Id: I047a8e7c4e519ef7c5deddaca0d1ad931dc91343