Fix harmful nesting spy on RootWindowContainer

Since ag/5842826, WindowManagerService instance under tests is a singleton
instance shared among all tests that extends WindowTestsBase class, creating
spied object on each test's @Before create chain of spied objects on
|mWm.mRoot| which may raise UnfinishedStubbingException at some points.

Bug: 122621913
Test: Pass all non-flaky presubmit tests on WM
  $ atest --test-mapping frameworks/base/services/core/java/com/android/server/wm
Change-Id: I2f215a24da14d3da42358af173796f4b8153ab91