Rewrite fsverity_init in C++ and load keys from keystore

Test: still see keys loaded from /product appears in /proc/keys
Test: Add X.509 DER cert files to keystore, see the key in
      /proc/keys after reboot
Bug: 112038744
Change-Id: I08006d8befa69e4bf416a2bed9e1813725877147
2 files changed