Check USB device/accessory perms with sys identity

As checking the permissions might call UsbDevice/UsbAccessory.getSerial
again, we have to make sure that in this code paths we run with the
system's identity so that we can always resolve the serial number.

Test: CtsVerifier UsbAccessory/UsbDevice test
Fixes: 117606509
Change-Id: I777be10b806ab02046f99805806ae1a16ebb0203