ANDROID: dm-android-verity: Rebase on top of 4.1

Following CLs in upstream causes minor changes to dm-android-verity target.
1. keys: change asymmetric keys to use common hash definitions
2. block: Abstract out bvec iterator
Rebase dm-android-verity on top of these changes.

Bug: 27175947

Signed-off-by: Badhri Jagan Sridharan <Badhri@google.com>
Change-Id: Icfdc3e7b3ead5de335a059cade1aca70414db415