fs_mgr: adding fs_mgr_get_slot_suffix() public API

The function returns "_a" or "_b" based on two possible values in
kernel cmdline:

  - androidboot.slot = a or b OR
  - androidboot.slot_suffix = _a or _b

Bug: 33254008
Bug: 36533366
Test: boot sailfish
Change-Id: Ia0a524e4145ebf61af5821f42ecad212c95ed748