Fix flaky test VibrationThreadTest#vibrate_multipleVibrations_withCancel

We're removing the callback verification since the test's focus is not on verifying the onComplete callback's timing after a vibration cancellation. Instead, we aim to confirm that the final vibration, triggered post-cancellation, actually executes as expected.

Fix: 368359597
Bug: 349156580
Test: atest --rerun-until-failure 1000 com.android.server.vibrator.VibrationThreadTest#vibrate_multipleVibrations_withCancel
Flag: TEST_ONLY
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:e52f0538606a48ef71319487344007cf5817e461)
Merged-In: I80dd468c9f31a1676e38b24766388934a6f03fa7
Change-Id: I80dd468c9f31a1676e38b24766388934a6f03fa7