WM: Fix flaky DragDropControllerTests tests

Stub Mockito spies using doReturn instead of when/then syntax. Using when/then
syntax on a spied object will call the actual implementation which is probably not
what we want for a stubbed function. Test failures suggest this or wrongly written
multi-threaded tests may be the root cause.

See: https://sponge.corp.google.com/target?id=aefd8278-1d23-458c-ab4c-56b19f24d745&target=com.android.frameworks.servicestests&searchFor=&show=FAILED&sortBy=STATUS

Test: FrameworksServicesTests:com.android.server.wm.DragDropControllerTests
Change-Id: Ib882f8d8602b848312631e69555a0316b9219b26