UsbService: Don't require permissions for UsbManager.getCurrentAccessory()

Permission check should only happen in openAccessory()
Otherwise an application will not be able to check for the current accessory
and ask for permissions (if it is a suitable match for the application)

BUG: 4069037

Change-Id: If5b44ebda2e8077598d96629163cc74aa336589e
Signed-off-by: Mike Lockwood <lockwood@android.com>