Export active dsu slot to system prop

The prop can be consumed by init scripts to do customized setup:

on property:ro.gsid.dsu_slot=oemtest
    # Do setup for test
on property:ro.gsid.dsu_slot=oemdemo
    # Do setup for demo

Bug: 277691885
Test: m
Change-Id: I7bd78b9ba31021b27d57c6f092dad5d7ebf6e59b
Merged-In: I7bd78b9ba31021b27d57c6f092dad5d7ebf6e59b
Cherry-picked from aosp/2588465