Compare the fingerprint before reading the partition

The update_verifier now compares the fingerprint of a partition before
performing the blocks read. If the fingerprint of the current system property
mismatches the one embedded in the care_map, verification of this partition
will be skipped. This is useful for the possible system only updates in the
future.

Bug: 114778109
Test: unit tests pass
Change-Id: Iea309148a05109b5810dfb533d94260d77ab8540