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: I0f8b27734caaa0f41f9c1b904d55af2112784a68
(cherry picked from commit 68658c0f)