Fix flaky VTS for vibration completion callback

Update assertions on vibration completion callback to use a fixed
timeout of 100ms. Tests are flaky on cuttlefish, which indicates it's
not hardware latency causing it.

Fix: 333639794
Test: atest VtsHalVibratorTargetTest
Change-Id: I986cdd8087895fc0694d689e70feef0a9fe4140e