Initialize fs-verity keys in shell script

This gives us two benefits:
  - Better compatibility to keyctl(1), which doesn't have "dadd"
  - Pave the way to specify key's security labels, since keyctl(1)
    doesn't support, and we want to avoid adding incompatible option.

Test: See keys loaded in /proc/keys
Bug: 128607724
Change-Id: Ia45f6e9dea80d037c0820cf1fd2bc9d7c8bb6302