Add new 'update_engine_log' GID

Add a new GID for controlling access to the update
engine logs.

This is an enabler for a new permission to grant
access update engine logs only:
  android.permission.READ_UPDATE_ENGINE_LOGS

Note: holders of android.permission.READ_LOGS will
still have access to update engine logs.

Bug: 397893018
Test: Flash software, check permissions of directory
    Flash software without this patch. Flash software with this
    CL on top. Confirm permission. Confirm log files included in
    bugreport.
Change-Id: I9632cf1b894a6deafd33a769f736b0736e8ca82e