ANDROID: dm: android-verity: Remove fec_header location constraint

This CL removes the mandate of the fec_header being located right
after the ECC data.

(Cherry-picked from https://android-review.googlesource.com/#/c/280401

)

Bug: 28865197
Signed-off-by: Badhri Jagan Sridharan <Badhri@google.com>
Change-Id: Ie04c8cf2dd755f54d02dbdc4e734a13d6f6507b5