Clear identity before calling deeper into USB code

This is needed as deeper in the code we call UsbAccessory.getSerial
which behaves differently depending on the caller. I.e. we have to make
sure to have the system server identity when we call this methods.

Fixes: 117547676
Test: - Started Android Auto
      - CtsVerifier USB Device and USB Accessory tests

Change-Id: I6d752ad5d2ff058dadf1e27e41247153c7db5ee6