Add support for USB Vendor ID in the add-ons.

The add-on manifest can now declare a USB vendor ID with the line
   usb-vendor=<id>
The ID must be a 16 hexadecimal value in the format 0xABCD.

The 'android' tool now has a new command 'android upate abd' that will create
a file called 'adb_usb.ini' in ~/.android/
This file will be read by adb to add support to those new vendor IDs.