Fix flaky WindowTokenClientController

Although the WindowTokenClientController can be created for testing, it
was still using the app's real IWindowManager. When override the
IWindowManager in test setup, it can be accessed by the real app before
the test case run.

Allow mock IWindowManager getter to make sure there is no unexpected
access from the real app.

Fix: 293161156
Test: atest FrameworksCoreTests:WindowTokenClientControllerTest
Change-Id: I784f365ffa17dbddefcce73e4a1ed9530f071be4