Fix a flaky test in FakeVehicleHalValueGeneratorsTest.

Fix a flaky test which might be caused by when the lock is waked
up, we might have more events than what we want. Furthermore, we
have to clear existing events after unregistering event generator.

Test: atest FakeVehicleHalValueGeneratorTest 10 times on cf_auto
Bug: 221084327
Change-Id: I65e87fa62b8538164fcdd60a9df871a9817f4f8e
Merged-In: I65e87fa62b8538164fcdd60a9df871a9817f4f8e
(cherry picked from commit 6ae468c1)