flashupdate: Use libhoth_error for key_rotation

libhoth commit ce5c230d64 (PR #266) and PR #276 replaced enum
key_rotation_err (KEY_ROTATION_CMD_SUCCESS) and int return codes with
libhoth_error (LIBHOTH_OK / LIBHOTH_ERR_FAIL) in protocol/key_rotation.h
and transports/libhoth_dbus.h.

Update key_rotate_helper.cpp and its unit test mocks to use
libhoth_error and LIBHOTH_OK.

Tested: Built flashupdate and obmc-phosphor-image against libhoth
5597928508.
Fusion-Link: fusion2 N/A (build fix for libhoth API update)
Google-Bug-Id: 493681862
Signed-off-by: Manan J. Mehta <mjmehta@google.com>
Change-Id: Ia8343362fdf53e7c85d8562c209d8baa01d4e4d8