Fix flaky VibratorManagerTest

The test wait condition for vibrate_withTriggerCallback_finishesVibration was reached too early and the test assertions would sometimes fail when checked too soon.

Also added an assertion to check the vibration actually ends by the callback, which is the main functionality being tested.

Change-Id: Iff014f36b6e235c428473a00bfd0c36819f63356
Fix: 193565275
Test: VibratorManagerTest