fs_mgr: validate corrected signatures

If signature verification fails and we have an error corrected
signature available, attempt to verify that instead.

Needs changes from
  Ie913c21ba1d07d6df4c6feeb7226b2ec963f4d19

Bug: 28943429
Change-Id: I7d48701916fe430b17aa05acb120f22a1802733d