Add uhid group to system_server

Ensure that uhid group (AID_UHID) is added to system_server, which would
allow it to read/write /dev/uhid and /dev/uinput nodes. This will allow
ATV devices to inject events.

Bug: 138311400
Test: none
Change-Id: Ie20fef5b5facff109bc4e068e648f335dd3a1e2c