Fix NPE on VibratorManagerService

The service fixes the VibrationAttributes extracted from the external
vibration AudioAttributes to apply usage and flags, and thus need to
check for permissions before applying them.

The check had a NPE error for unknown external vibrations.

Fix: 227677233
Test: VibratorManagerServiceTest
Change-Id: I864a0538c90d65b96ec319ce75cb64f962138bb9