Stop previous notification vibration on new buzzy notification

 VibratorHelper needs to explicitly cancel a previously playing
non-insistent notification vibration,
 otherwise a looping vibration will not be stopped when a new notification with vibration is posted.

Flag: EXEMPT bugfix
Test: atest NotificationAttentionHelper
Bug: 340653186
Change-Id: I4a6730faa89ea7f2c176512c6decb03a44d858a4