Add post-install verification for BBOTAs

Similar to the assertations in file-based OTA, we perform verification
for block-based OTAs (BBOTAs) after updating a partition, for both of
the incremental and full OTAs. It increases the update time (~20s on
Nexus 6), but will capture unnoticed errors right away.

Bug: 21500869
Change-Id: I89ac8fe623b855721b7affd07cf9426a23433ab0