Forward Activity stop signal to attached windows.

Simply put there is nothing ensuring PopupWindow's stop using
the Surface before the app is stopped. Since PopupWindow's are
detached from the main View Hierarchy teardown of the main window
provides no such guarantee. It seems enough to forward the "WindowStopped"
signal from ViewRoot to other ViewRoot's using the token of that ViewRoot.

Bug: 62536731
Test: Manual
Change-Id: Ib431f49be7e06a6397a40c11ec7e842a8bc5caf9