Allow overriding ro.debuggable to 1 on USER builds

When init found "/force_debuggable" in the first-stage ramdisk, it will
do the following if the device is unlocked:
  1. load /system/etc/adb_debug.prop (with ro.debuggable=1)
  2 .load userdebug_plat_sepolicy.cil instead of original plat_sepolicy.cil from
    /system/etc/selinux/.

This make it possible to run VTS on a USER build GSI, by using a special
ramdisk containing "/force_debuggable".

Bug: 126493225
Test: unlock a USER build device, check 'adb root' can work
Change-Id: I9b4317bac1ce92f2c0baa67c83d4b12deba62c92