fs_mgr: support different dm-verity error modes

AVB is going to support different modes to handle dm-verity errors.
See the following CL for more details:

  - https://android-review.googlesource.com/#/c/392873/

The verity mode is controlled by bootloader through androidboot.veritymode in
kernel command line. fs_mgr should read the value from there and specify the
corresponding flag when loading dm-verity table into kernel.

Also removes some unused #include libraries.

Bug: 38157502
Test: Manually tested different dm verity modes:
      - "restart_on_corruption" (androidboot.veritymode=enforcing)
      - "ignore_corruption" (androidboot.veritymode=logging)
      - None, default mode is EIO in kernel (androidboot.veritymode=eio)

Change-Id: I80e1e817a148b54fb67ba58112d376dc2cf37c98