Check partition hash before resorting to block_image_verify

It's not necessary to call block_image_verify for the most common
case where we attempt an update for the first time and the target
partition has expected contents. It is sufficient to confirm that
the target partition is intact.

Computing a hash is much faster than simulating the update, which
means this change should make the update speed for v3 much closer
to v2 for the common case. If the update was interrupted, this is
going to make resuming somewhat slower, of course. Hopefully this
should be rather rare.

Bug: 20309033
Change-Id: I04792f3681bf2cd5d9a4e1c41eb883c7f26c6f58