Fix widget view memory leak on hub

We currently don't remove listeners from AppWidgetHost, meaning that the
AppWidgetHostViews leak after the hub has been closed.

This change ensures that we correctly remove listeners when the hub is
closed.

Fixes: 406044666
Test: atest CommunalAppWidgetViewModelTest
Flag: EXEMPT bugfix
Change-Id: I0b27572ff7e24a803d40058bfbf66be449022654