fs_mgr: update to new androidboot.slot param

androidboot.slot_suffix is being deprecated for a androidboot.slot.
Bootloader must pass slot used to boot into the androidboot.slot which
does not include the '_' character

Test: boot android
Bug: 36533366
Change-Id: I6137bd08418f67a88120c8609eda10b2ada4607d
Signed-off-by: Fernando Lugo <flugo@google.com>