Set return value for mock(IWindow).asBinder() to prevent exception

In TestWindowManagerPolicy#addSplashScreen(), mock(IWindow) is
created and it's passed to the constructor of WindowState. In the
constructor, c.asBinder().linkToDeath() is called, but currently,
c.asBinder() returns null, which ends up throwing an exception.

This CL sets a proper return value for c.asBinder(). With this CL,
all the tests in AppWindowContainerControllerTests pass in ARC.

Bug: 110906840
Test: atest AppWindowContainerControllerTests
Change-Id: I5cacd6635eb11f174d14c7c2a3873dc648d43527