Use singleton WindowManagerService for testing

This CL introduces a single WindowManagerService instance and uses it
for all tests that extend WindowTestBase. This way we can decrease the
number of object counts in the heap after running 526 presubmit tests
in WmTests.

Bug: 113239988
Test: Pass all presumit tests in WmTests.
  $ atest --test-mapping frameworks/base/services/core/java/com/android/server/wm

Change-Id: I8e5af477def6207bcc86cace6b240873516d075d