fs_mgr: Update for new libavb API.

In https://android-review.googlesource.com/#/c/392873/ we slightly
changed the libavb API. This CL updates fs_mgr to use the new API.

Bug: 38157502
Test: Manually tested.
Change-Id: I90484a3d045c9d51946c14e8136258d2f940da81
3 files changed