DO NOT MERGE: Switch fs_mgr to use SHA-256 instead of SHA-1

Verity metadata signatures will be switched to SHA-256. Switch
fs_mgr signature verification to use the correct algorithm.

Needs matching changes from
  https://googleplex-android-review.git.corp.google.com/#/c/579905/
  https://googleplex-android-review.git.corp.google.com/#/c/583213/
  https://googleplex-android-review.git.corp.google.com/#/c/583214/
  https://googleplex-android-review.git.corp.google.com/#/c/583233/

Bug: 15984840
Bug: 18120110
Bug: 17917515
Change-Id: I8f90519bffa105a0eb7abeaad3aea1ffceb851e2
(cherry picked from commit a3465e25)