libfs_avb: Allow overriding the slot suffix.

There is no easy way to override fs_mgr_get_slot_suffix for testing or
for simulating slot switches. It relies on global properties, and
since fstab is usually a static library, linked multiple times, we can't
just stick a global variable in fs_mgr_slotselect.

libfs_avb already takes slot suffix arguments in various places. This CL
extends that by adding arguments to AvbHandle and FsManagerAvbOps.

Bug: N/A
Test: snapshotctl
Change-Id: Ide7f052b38249d6bc79a6a2c124baf8ca08bcc3b