FP4: Store SPL only if locked and on Stock ROM

* Rollback Index == SPL only if we:
  - Are on the Stock ROM
    (Non Stock OS-es spoof SPL values)
  - Have a LOCKED bootloader
    (A device with an UNLOCKED bootloader MAY NOT have SPL == Rollback Index)

* On a given device from the wild, which can only be interfaced via ADB
  and Fastboot, this is the only way to understand the Rollback Index and
  prevent AVB Red state on LOCKING the bootloader