Expose error code for RoR reboot system API

The clients of resume on reboot want additional information for
reboot failures, so they can handle these failures better.
Therefore expose the reboot failure error code via SystemApi.

Bug: 183646725
Test: atest FrameworksServicesTests:RecoverySystemServiceTest;
      Do an RoR with gmscore
Change-Id: Ia02655218194a070ce02e9dae6fd62e19259c9f9