Fix the NPE in LockedUserStateTest since mocks gets cleared before the test finishes execution.

After ag/31327061 is merged, mocks get cleared in tests. In some
cases, mocks are cleared before the executors are completed. So, in
teardown, wait for the executors to complete.

Bug: 390716213
Test: atest and android studio builds
Flag: TEST_ONLY
Change-Id: Ie90b0f653d1619180cf177a1010aa646c97fac92