Report the true value of more RoR metrics

Design ErrorCode for errors of loadRebootEscrowData. Also report
the status of vbmeta digest, as we plan to associate k_k with
vbmeta in the future.

Bug: 179105110
Test: atest FrameworksServicesTests:RebootEscrowManagerTests
Change-Id: I07168a7bc4fd20d41df9b87136e1e5158129d7f6